Re: fpga: fpga_mgr_get() buggy ?

[Federico Vaga]

Hi alan,

inline comments

On Wednesday, August 15, 2018 11:02:12 PM CEST Alan Tull wrote:
> On Wed, Jul 18, 2018 at 4:47 PM, Federico Vaga <federico.vaga@xxxxxxx> 
wrote:
> > Hi Alan,
> > 
> > Thanks for your time, comments below
> > 
> > On Wednesday, July 18, 2018 9:47:24 PM CEST Alan Tull wrote:
> >> On Thu, Jun 28, 2018 at 2:50 AM, Federico Vaga <federico.vaga@xxxxxxx>
> > 
> > wrote:
> >> > On Wednesday, 27 June 2018 23:23:07 CEST Alan Tull wrote:
> >> >> On Wed, Jun 27, 2018 at 4:25 AM, Federico Vaga
> >> > 
> >> > <federico.vaga@xxxxxxx> wrote:
> >> >> > Hi Alan,
> >> >> > 
> >> >> > On Tuesday, 26 June 2018 23:00:46 CEST Alan Tull wrote:
> >> >> >> On Fri, Jun 22, 2018 at 2:53 AM, Federico Vaga
> >> >> >> <federico.vaga@xxxxxxx> wrote:
> >> >> >> 
> >> >> >> Hi Federico,
> >> >> >> 
> >> >> >> >> > What is buggy is the function fpga_mgr_get().
> >> >> >> >> > That patch has been done to allow multiple FPGA manager
> >> >> >> >> > instances
> >> >> >> >> > to be linked to the same device (PCI it says). But function
> >> >> >> >> > fpga_mgr_get() will return only the first found: what about
> >> >> >> >> > the
> >> >> >> >> > others?
> >> 
> >> Looking at this further, no code change is needed in the FPGA API to
> >> support multiple managers.  A FPGA manager driver is a self-contained
> >> platform driver.  The PCI driver for a board that contains multiple
> >> FPGAs should create a platform device for each manager and save each
> >> of those devs in its pdata.
> >> 
> >> >> >> >> > Then, all load kernel-doc comments says:
> >> >> >> >> > 
> >> >> >> >> > "This code assumes the caller got the mgr pointer from
> >> >> >> >> > of_fpga_mgr_get() or fpga_mgr_get()"
> >> >> >> >> > 
> >> >> >> >> > but that function does not allow me to get, for instance,
> >> >> >> >> > the
> >> >> >> >> > second FPGA manager on my card.
> >> 
> >> fpga_mgr_get() will do what you want if your PCI driver creates a
> >> platform device per FPGA manager as mentioned above.
> >> 
> >> >> >> >> > Since, thanks to this patch I'm actually the creator of the
> >> >> >> >> > fpga_manager structure,  I do not need to use fpga_mgr_get()
> >> >> >> >> > to
> >> >> >> >> > retrieve that data structure.
> >> 
> >> The creator of the FPGA mgr structure is the low level FPGA manager
> >> driver, not the PCIe driver.
> > 
> > In my case it is.
> > It's a bit like where SPI driver is the low level FPGA manager driver for
> > the xilinx-spi.c. But if I understand what you mean, I should always have
> > a
> > platform_driver just for the FPGA manager even if it has a 1:1 relation
> > with its carrier? In other words, I write two drivers:
> > - one for the FPGA manager
> > - one for the PCI device that then register the FPGA manager driver
> > 
> > In my case the FPGA programmed is also the PCIe bridge (GN4124). Probably
> > I
> > can do it anyway, because the part dedicated to FPGA programming is quite
> > independent from the rest (don't remember all details)
> > 
> >> >> >> >> > Despite this, I believe we still need to increment the
> >> >> >> >> > module
> >> >> >> >> > reference counter (which is done by fpga_mgr_get()).
> >> 
> >> This is only done in the probe function of a FPGA region driver.
> >> 
> >> >> >> >> > We can fix this function by just replacing the argument from
> >> >> >> >> > 'device' to 'fpga_manager' (the one returned by create() ).
> >> >> >> >> 
> >> >> >> >> At first thought, that's what I'd want.
> >> >> >> >> 
> >> >> >> >> > Alternatively, we
> >> >> >> >> > can add an 'owner' field in "struct fpga_manager_ops" and
> >> >> >> >> > 'get'
> >> >> >> >> > it
> >> >> >> >> > when we use it. Or again, just an 'owner' argument in the
> >> >> >> >> > create()
> >> >> >> >> > function.
> >> >> >> >> 
> >> >> >> >> It seems like we shouldn't have to do that.
> >> >> >> > 
> >> >> >> > Why?
> >> >> >> 
> >> >> >> OK yes, I agree; the kernel has a lot of examples of doing this.
> >> >> >> 
> >> >> >> I'll have to play with it, I'll probably add the owner arg to the
> >> >> >> create function, add owner to struct fpga_manager, and save.
> >> >> > 
> >> >> > I have two though about this.
> >> >> > 
> >> >> > 1. file_operation like approach. The onwer is associated to the
> >> >> > FPGA manager operation. Whenever the FPGA manager wants to use an
> >> >> > operation it get/put the module owner of these operations
> >> >> > (because this is what we need to protect). With this the user is
> >> >> > not directly involved, read it as less code, less things to care
> >> >> > about. And probably there is no need for fpga_manager_get().
> >> >> 
> >> >> How does try_module_get fit into this scheme?  I think this proposal
> >> >> #1 is missing the point of what the reference count increment is
> >> >> meant to do.  Or am I misunderstanding?  How does that keep the
> >> >> module from being unloaded 1/3 way through programming a FPGA?
> >> >> IIUC you are saying that the reference count would be incremented
> >> >> before doing the manager ops .write_init, then decremented again
> >> >> afterwards, incremented before each call to .write, decremented
> >> >> afterwards, then the same for .write_complete.
> >> > 
> >> > I'm not saying to do module_get/put just around the mops->XXX(): it's
> >> > too much. Just where you have this comment:
> >> > 
> >> > "This code assumes the caller got the mgr pointer from
> >> > of_fpga_mgr_get() or fpga_mgr_get()"
> >> > 
> >> > Because, currently, it's here where we do module_get()
> >> 
> >> No it's not.
> > 
> > It is not in the code, but the comment says that we should do it before
> > calling fpga_mgr_load()
> > 
> >> fpga_mgr_get() or of_fpga_mgr_get() is called when the region is
> >> created such as in of-fpga-region's probe.  That way, as long as the
> >> region exists, the manager won't be unloaded.  If someone wants to
> >> start unloading modules, they need to unload higher level ones first,
> >> so they'll have to unload the region before mgr.
> >> 
> >> > Most mops are invoked within a set of static functions which are
> >> > called only by few exported functions. I'm suggesting to do
> >> > module_get/put in those exported function at the beginning (get) and
> >> > and the end (put) because we know that within these functions, here
> >> > and there, we will use mops which are owned by a different module.
> >> > We do not want the module owner of the mops to disappear while someone
> >> > is doing fpga_mgr_load(). For example, inside fpga_mgr_load() we use
> >> > most of the mops, so we 'get' the module at the beginning and 'put' it
> >> > at the end. The same for fpga_region_program_fpga().
> >> 
> >> If we do it the way you are suggesting, then someone can unload the
> >> manager module without unloading the region.  The region code will be
> >> in for a nasty surprise when programming is attempted.
> > 
> > Of course, this should be taken into account. I was not suggesting a
> > precise implementation, but only the idea to hide get/put. Probably there
> > other things as well that I'm not considering (indeed I do not have a
> > patch, but just a comment)
> > 
> >> > Like this we do not have to ask users to do fpga_mgr_get()/put().
> >> 
> >> The only user who has to do this is a region's probe function.
> >> 
> >> I'm assuming that only fpga_region is using fpga_mgr_load() and
> >> anybody who is creating a region uses fpga_region_program_fpga().
> >> That's what I want to encourage anyway.  I should probably move
> >> fpga_mgr_load to a private header.
> > 
> > All right, if this is what you want to encourage I do not have anything to
> > say because I did exactly what you do not want to encourage :)
> > 
> > For me this change everything because I do not use regions since I do not
> > have them. The way the API is exposed to me did not encouraged me to use
> > their API. In addition, the documentation guides me directly to the FPGA
> > manager.
> 
> The documentation says:
> 
> "If you are adding a new interface to the FPGA framework, add it on
> top of a FPGA region to allow the most reuse of your interface."

I would change this in something like:

"If you are adding a new interface to the FPGA framework, add it on
top of a FPGA region."

What I understand from the current statement is:
"if you care about re-usability, add your interface on top of an FPGA region" 
Since in my special case, I do not care about re-usability, I skipped the FPGA 
region.

> https://www.kernel.org/doc/html/latest/driver-api/fpga/intro.html
> 
> But the stuff that I submitted yesterday goes back through the docs to
> clear out anything that is not clear about this.
> 
> > To be honest I did not have much time to look at the region code. My
> > understanding, after a quick look, is that it works great with
> > device-tree.
> 
> It is used by the DFL framework which doesn't use device tree.
> 
> > But what if I do not have it? Actually, I cannot use device-tree because
> > of
> > environment limitations and Linux version in use. Oops, this implies that
> > I
> > back-ported the FPGA manager to an older Linux version? Yes, guilty :)
> 
> I separated region code from its device-tree dependencies.  But if you
> can't use device-tree, then you end up having to implement some of the
> things DT gives you for free.

unfortunately yes

> > Anyway, I'm using the API exposed, and if part of the assumptions behind
> > this API is that I should use device-tree, then I'm out of scope.
> > 
> > <chatting>
> > Just for chatting. One addition I made for the FPGA manager is to support
> > 
> > dynamic loading of FPGA code using char devices. Something like:
> >    dd if=binary.bin of=/dev/fpga0
> >    cat binary.bin > /dev/fpga0
> 
> Since it's not handling the bridge, there's some risk involved.
> 
> > We do not have device tree, and we do not have binaries in /lib/firmware.
> > It's quite handy to quickly load a binary just synthesized, especially for
> > debugging purpose.
> 
> Like a debugfs?
> 
> https://lkml.org/lkml/2018/8/2/125
> 
> But for a debugging/developing, I have a debugfs that was a downstream
> patchset that I'm cleaning for submission.

Yes, like that more or less.

I did a chardevice in /dev/ because in our environment debugfs is not mounted 
automatically and we need this feature also operationally. But yes, that's the 
idea.

One comment. The code on github [1] looks like it is assuming that on 
userspace people write the full image in one shot and it is smaller that 4MiB. 
What I did is to build a scatterlist in order to support the following case:

cat image.bin > /sys/kernel/debug/.../image

where `cat` write 4K at time. And it can be bigger than 4M

[1] https://github.com/altera-opensource/linux-socfpga/blob/socfpga-4.17/
drivers/fpga/fpga-mgr-debugfs.c

Below (end of this email) the patch I quickly did last year. Unfortunately I 
do not have the time to clean this solution, so it is the very first thing I 
implemented without many thoughts (that's why I never pushed this patch).


> > If you are interested I can try to clean it up (at some
> > point, probably in autumn), or I can show you the code in private for a
> > quick look.
> > </chatting>
> > 
> >> > <Parenthesis>
> >> > fpga_region_program_fpga() does not do (today) fpga_mgr_get() because
> >> > it assumes, but it is not enforced, that both fpga_region and fpga_mgr
> >> > are child of the same device.
> >> 
> >> fpga_region_program_fpga() doesn't do fpga_mgr_get() becuase
> >> fpga_mgr_get() happens when the fpga_region probes.  The assumption I
> >> am making is that nobody other than a region is calling
> >> fpga_manager_load().  I should create a fpga_private.h and move
> >> fpga_manager_load() out of fpga-mgr.h to make that official.
> > 
> > Yes, I agree. If what I'm doing is not supposed to happen I should not be
> > allowed to do it :)
> > 
> > <suggestion>
> > If you want to encourage people to use regions rather than the manager
> > directly, have you though about changing the API and merge in a single
> > module fpga-mgr and fpga-region?
> > 
> > Brainstorming. Perhaps, it is possible to have a `struct fpga_region_info`
> > and when we register and FPGA manager we use something like:
> > 
> > struct fpga_manager *fpga_mgr_create(struct device *dev, const char *name,
> > 
> >                                      const struct fpga_manager_ops *mops,
> >                      
> >                      struct fpga_region_info *info, int n_regions,
> >                      
> >                                      void *priv)
> > 
> > So those regions will be created directly and the interface will be
> > smaller
> > and easier.
> > 
> > Don't waste much time on this suggestion, as I said before I did not study
> > much the fpga-region.c code and perhaps this is not possible and I'm just
> > speaking rubbish :)
> > </suggestion>
> > 
> >> > Probably this is true 99.99% of the
> >> > time.
> >> > Let me open in parallel another point: why fpga_region is not a child
> >> > of fpga_manager?
> >> 
> >> FPGA regions are children of FPGA bridges.
> >> 
> >> Alan
> > 
> > --
> > Federico Vaga
> > [BE-CO-HT]
> > 
> > 
> > --
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