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

[Srinivas Kandagatla <srinivas.kandagatla@oss.qualcomm.com>]

On 9/9/26 8:53 AM, Ekansh Gupta wrote:
>> 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.
>>
>> 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.
>>
>>
>> But this does bring to a question which the cover letter should explain
>> - but doesn't: what problem does this patch series actually solve?
>>
>> Regards,
>> Bjorn
> Agreed. I'll target #2: QDA implementing the existing fastrpc UABI
> alongside the new one, rather than a driver split by platform.
> 
> On how to get there: the blocker we hit in v1 was that legacy fastrpc
> buffer semantics appeared to need a drm_file, and there's no exported
> way to construct one outside the DRM core. I want to re-examine that
> constraint rather than treat it as final, since the legacy interface's
> own buffer model (a dma_buf fd as the buffer identity, no GEM
> involved) is not inherently tied to drm_file, that's how the existing
> misc driver implements it today. I don't have a concrete design yet
> and would rather work through it here than commit to one prematurely.
> If anyone has thoughts on how the legacy UABI could be served without
> requiring a drm_file per session or if I can somehow bind drm_file with
> chardev by exposing some APIs from DRM core, I'd welcome them.
> 
> On the cover letter: fair point, and I'll fix it. The problem this
> series solves is that a miscdevice interface requires us to hand-roll
> what the accel/DRM subsystem already provides as common
> infrastructure: GEM for buffer lifecycle and reference counting,
> PRIME for cross-driver import/export, per-file (per-open) context and
> handle-namespace isolation, and the existing debug and lifecycle
> tooling the DRM core already ships. Every accelerator driver added to
> drivers/accel (habanalabs, ivpu, qaic, rocket) has taken this path for
> the same reason, rather than each maintaining its own equivalent
> inside drivers/misc. Building QDA directly on this shared
> infrastructure, instead of extending fastrpc's own ad hoc buffer and
> session tracking to cover the same ground, avoids that duplication
Am sure you must have already tried this, but Can you not migrate
existing fastrpc driver to this shared infrastructure under the hood?

Can you elaborate on what are the blockers you hit in doing so?

This will ensure that UAPI is retained and still get benefit of QDA.

--srini
> going forward.