Re: [PATCH 3/8] of/fdt: add function to get the SoC wide DMA addressable memory size

[Nicolas Saenz Julienne]

On Thu, 2019-08-08 at 09:02 -0600, Rob Herring wrote:
> On Tue, Aug 6, 2019 at 12:12 PM Nicolas Saenz Julienne
> <nsaenzjulienne@xxxxxxx> wrote:
> > Hi Rob,
> > 
> > On Mon, 2019-08-05 at 13:23 -0600, Rob Herring wrote:
> > > On Mon, Aug 5, 2019 at 10:03 AM Nicolas Saenz Julienne
> > > <nsaenzjulienne@xxxxxxx> wrote:
> > > > Hi Rob,
> > > > Thanks for the review!
> > > > 
> > > > On Fri, 2019-08-02 at 11:17 -0600, Rob Herring wrote:
> > > > > On Wed, Jul 31, 2019 at 9:48 AM Nicolas Saenz Julienne
> > > > > <nsaenzjulienne@xxxxxxx> wrote:
> > > > > > Some SoCs might have multiple interconnects each with their own DMA
> > > > > > addressing limitations. This function parses the 'dma-ranges' on
> > > > > > each of
> > > > > > them and tries to guess the maximum SoC wide DMA addressable memory
> > > > > > size.
> > > > > > 
> > > > > > This is specially useful for arch code in order to properly setup
> > > > > > CMA
> > > > > > and memory zones.
> > > > > 
> > > > > We already have a way to setup CMA in reserved-memory, so why is this
> > > > > needed for that?
> > > > 
> > > > Correct me if I'm wrong but I got the feeling you got the point of the
> > > > patch
> > > > later on.
> > > 
> > > No, for CMA I don't. Can't we already pass a size and location for CMA
> > > region under /reserved-memory. The only advantage here is perhaps the
> > > CMA range could be anywhere in the DMA zone vs. a fixed location.
> > 
> > Now I get it, sorry I wasn't aware of that interface.
> > 
> > Still, I'm not convinced it matches RPi's use case as this would hard-code
> > CMA's size. Most people won't care, but for the ones that do, it's nicer to
> > change the value from the kernel command line than editing the dtb.
> 
> Sure, I fully agree and am not a fan of the CMA DT overlays I've seen.
> 
> > I get that
> > if you need to, for example, reserve some memory for the video to work, it's
> > silly not to hard-code it. Yet due to the board's nature and users base I
> > say
> > it's important to favor flexibility. It would also break compatibility with
> > earlier versions of the board and diverge from the downstream kernel
> > behaviour.
> > Which is a bigger issue than it seems as most users don't always understand
> > which kernel they are running and unknowingly copy configuration options
> > from
> > forums.
> > 
> > As I also need to know the DMA addressing limitations to properly configure
> > memory zones and dma-direct. Setting up the proper CMA constraints during
> > the
> > arch's init will be trivial anyway.
> 
> It was really just commentary on commit text as for CMA alone we have
> a solution already. I agree on the need for zones.

Ok, understood :)

> > > > > IMO, I'd just do:
> > > > > 
> > > > > if (of_fdt_machine_is_compatible(blob, "brcm,bcm2711"))
> > > > >     dma_zone_size = XX;
> > > > > 
> > > > > 2 lines of code is much easier to maintain than 10s of incomplete code
> > > > > and is clearer who needs this. Maybe if we have dozens of SoCs with
> > > > > this problem we should start parsing dma-ranges.
> > > > 
> > > > FYI that's what arm32 is doing at the moment and was my first instinct.
> > > > But
> > > > it
> > > > seems that arm64 has been able to survive so far without any machine
> > > > specific
> > > > code and I have the feeling Catalin and Will will not be happy about
> > > > this
> > > > solution. Am I wrong?
> > > 
> > > No doubt. I'm fine if the 2 lines live in drivers/of/.
> > > 
> > > Note that I'm trying to reduce the number of early_init_dt_scan_*
> > > calls from arch code into the DT code so there's more commonality
> > > across architectures in the early DT scans. So ideally, this can all
> > > be handled under early_init_dt_scan() call.
> > 
> > How does this look? (I'll split it in two patches and add a comment
> > explaining
> > why dt_dma_zone_size is needed)
> > 
> > diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
> > index f2444c61a136..1395be40b722 100644
> > --- a/drivers/of/fdt.c
> > +++ b/drivers/of/fdt.c
> > @@ -30,6 +30,8 @@
> > 
> >  #include "of_private.h"
> > 
> > +u64 dt_dma_zone_size __ro_after_init;
> 
> Avoiding a call from arch code by just having a variable isn't really
> better. I'd rather see a common, non DT specific variable that can be
> adjusted. Something similar to initrd_start/end. Then the arch code
> doesn't have to care what hardware description code adjusted the
> value.

Way better, I'll update it.

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