Re: [PATCH v2 00/15] accel/qda: Qualcomm DSP Accelerator driver

[Dmitry Baryshkov]

On Tue, Sep 08, 2026 at 05:44:10PM -0500, Bjorn Andersson wrote:
> On Wed, Aug 26, 2026 at 06:37:00PM +0530, Ekansh Gupta wrote:
> > On 20-08-2026 20:17, Rob Clark wrote:
> > > On Wed, Aug 19, 2026 at 11:15 PM Krzysztof Kozlowski <krzk@xxxxxxxxxx> wrote:
> > >>
> > >> On 19/08/2026 17:48, Rob Clark wrote:
> > >>> On Wed, Aug 19, 2026 at 8:27 AM Krzysztof Kozlowski <krzk@xxxxxxxxxx> wrote:
> > >>>> The rule of usptream development is that we do not accept duplicated
> > >>>> code, just because a vendor wants to write something new. This is
> > >>>> basically the concept applied all over the drivers tree, where we pushed
> > >>>> back against all sorts of duplications all over the vendors.
> > >>>>
> > >>>> What I miss in this thread is why would there be any exception here. We
> > >>>> do not grant exceptions from standard practices on "I want" reasons.
> > >>>
> > >>> I agree that we should not have duplicated drivers just for vendor
> > >>> lolz.  But when it comes to adopting common frameworks and integrating
> > >>> better into the ecosystem, this doesn't seem like something we should
> > >>> actively discourage.  I don't think this is a case of vendor lolz, but
> > >>
> > >> No one discourages it. Following standard Linux kernel practices and
> > >> requirements is not discouraging, do not twist the narrative here.
> > >> Again, it is standard upstream review telling that we do not duplicate
> > >> drivers. Ever, unless there is serious exception needed.
> > > 
> > > I wasn't trying to twist the narrative, just trying to come up with a
> > > path forward that isn't "no" or "improve existing driver", since
> > > neither of those gets us towards a future using common frameworks.
> > > 
> > >> I asked why there should be an exception granted? Is the reason for
> > >> exception following:
> > >> "We want to adopt common framework"
> > >> ?
> > > 
> > > Possibly?  But I don't think we want two drivers to be any sort of
> > > long term solution.  (Ie. as long as venus/iris have co-exist.)
> > > 
> > >>
> > >>> rather reacting to drm/accel emerging as the standard framework for
> > >>> this sort of driver.
> > >>>
> > >>> So how do we get from here to there?
> > >>
> > >> What is wrong with my proposal?
> > > 
> > > Maybe I missed something, my understanding was your proposal was
> > > "Grow/replace/improve existing driver instead of coming with a
> > > duplicate"..  grow or improve doesn't move us toward common
> > > frameworks.  Maybe "replace" is a valid option.  If there is something
> > > I missed, then I apologize.
> > > 
> > > Options I can think of are:
> > > 
> > > 1. Hardware cutoff.. new hw gets new driver, existing hw gets existing
> > >    driver
> > > 2. Backwards compat chardev registered by new driver, providing existing
> > >    UABI.  I'm not 100% sure about the feasibility/drawbacks of this..
> > >    AFAIU the fastrpc folks where planning a backwards compat layer in
> > >    userspace, so maybe it is possible.
> > > 3. exception?
> > > 
> > > I'd like to know what the feasibility of #2 is, since at a high level
> > > that sounds like the best option.  Possibly limit exposure of legacy
> > > UABI to existing hw so we don't get into a place of needing to extend
> > > the legacy UABI for new hw?
> > > 
> > > But #1 sounds like a non-controversial place to start regardless.
> > > Possibly with #2 coming as followup and necessary step before eventual
> > > migration to new driver for existing hw?
> > > 
> > > Even if we start with #2, how do we handle first-merge-window
> > > bugs/regressions without reverting addition of new driver and removal
> > > of old?  It seems like we'd need a window of a couple release cycles
> > > where both drivers exist?
> > > 
> > > Maybe others have other/better options in mind?
> > To all, I'm seeking on the approach I should follow to go ahead here. I
> > can work on implementing #1(as per Rob's list) with hw specific
> > compatible for v4 if it's acceptable.
> > 
> 
> I don't see any reason for you to define a "hw specific compatible",
> because as you have shown in this series (and as Rob point out), there's
> no difference in the "hardware".
> 
> The only reason for your "hw specific compatible" is to make a software
> selection in Linux - and that's not what DeviceTree is for.

That's not exactly true. There are protocol differences. For example,
polling mode is supported only since a certain timeline in the history.
Likewise other interface features are not supported on all the FastRPC
devices. For the polling mode support we were already beaten by the lack
of SoC-specific compats.

> As such, I don't see that you have a DeviceTree problem at all, because
> this is a Linux-internal problem.
> 
> > #2(compat driver) is something that we are still exploring as we
> > couldn't find any standard way to achieve it. We might start a separate
> > discussion for that once we have few possible designs with us.
> > 
> 
> This is the actual problem!
> 
> We have existing user space that depends on the ioctl interface exposed
> by the current misc driver. You must not break these.

This is clear.

> Hardware cutoff is not a viable solution, because that's just a
> declaration that we'll let the old platforms rotten - or alternatively
> you commit to maintain two drivers to the very same feature and quality
> level.
> 
> So the only reasonable solution is #2; from there it's a valid question
> if you reach that point my stepwise migrating the current misc driver
> that solution, or if you present a new driver with the fully backwards
> compatible interface, alongside the new ABI.

I think the general direction was #3 (or #2.1): implement a shim layer
on top of the QDA driver as a separate module. Put all the historical
over-complicated solutions into that shim module and let it die at some
point. Current fastrpc driver lets userspace specify buffers in several
different ways, forcing the kernel driver to perform a lot of work
with buffer addresses. I don't think that this legacy code should be a
part of the QDA. It can go to qda-fastrpc-shim, keeping it out of the
kernel memory if it's not necessary.

> But this does bring to a question which the cover letter should explain
> - but doesn't: what problem does this patch series actually solve?

I agree that it should be a part of the cover letter.

As a person who triggered this work, I can propose my reasons:

- Current driver has over-complicated memory manager (both on the
  userspace and on the kernel side). Correspondng uAPI is not really
  suitable for virtualization. Using GEM simplifies both the kernel
  driver and uAPI. Also using handle-offset-length to specify the
  buffers makes it easy to support virtual QDA devices.

- Current driver predates the accel subsystem. Using common subsystem
  simplifies reviews. The QDA driver has gotten several comments about
  the usage of the DMA-BUFs. It'not unlikely that the same issues are
  present in the current FastRPC driver, just being unnoticed.

- The ideas present in the current FastRPC driver also predate the
  current design practices. The uAPI was created in the ad-hoc way, just
  following the momentary needs. Driver code also shows the result of
  that, having enough of the spaghetti code.

Given all of that, yes, it is possible to provide an evolution of the
FastRPC driver into the accel+shim, improve the code quality meanwhile
and end up with the good enough split. However I think that the path
taken would be longer and the net result might be worse.

With all of that in mind, my suggestion is to continue working on the
QDA driver, get the core of it to integrate nicely with the accel and
DMA-BUF usage requirements and implement the (minimal?) fastrpc shim on
top of it.


-- 
With best wishes
Dmitry