linux-kernel - Re: [PATCH 09/13] platform/raspberrypi: Add new vc-sm-cma driver

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Message-ID: <79d45a6d-df37-475a-9edd-57e7a06155e3@kernel.org>
Date: Mon, 3 Nov 2025 15:00:11 +0100
From: Krzysztof Kozlowski <krzk@...nel.org>
To: Jai Luthra <jai.luthra@...asonboard.com>,
 Florian Fainelli <florian.fainelli@...adcom.com>,
 Raspberry Pi Kernel Maintenance <kernel-list@...pberrypi.com>,
 Dave Stevenson <dave.stevenson@...pberrypi.com>,
 bcm-kernel-feedback-list@...adcom.com
Cc: Phil Elwell <phil@...pberrypi.com>, Stefan Wahren <wahrenst@....net>,
 Laurent Pinchart <laurent.pinchart@...asonboard.com>,
 Kieran Bingham <kieran.bingham@...asonboard.com>,
 Sumit Semwal <sumit.semwal@...aro.org>,
 Christian König <christian.koenig@....com>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 Dom Cobley <popcornmix@...il.com>,
 Alexander Winkowski <dereference23@...look.com>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Juerg Haefliger <juerg.haefliger@...onical.com>
Subject: Re: [PATCH 09/13] platform/raspberrypi: Add new vc-sm-cma driver

On 03/11/2025 14:49, Jai Luthra wrote:
> Hi Krzysztof,
> 
> 
> Thanks for the review.
> 
> Quoting Krzysztof Kozlowski (2025-11-02 15:00:58)
>> On 31/10/2025 18:27, Jai Luthra wrote:
>>> From: Dave Stevenson <dave.stevenson@...pberrypi.com>
>>>
>>> Add Broadcom VideoCore Shared Memory support.
>>
>> You will have to come with really, really good argument why CMA is not
>> working for you. This is how you write commit msgs. All further is not
>> really helpful.
>>
> 
> From my limited understanding, this driver allows sharing buffers between
> the kernel and the firmware that controls the ISP and codec on the remote
> VideoCore VPU.
> 
> Maybe Dave can comment better on the historical reasons of this approach
> versus other DMA buffer sharing mechanisms available in the kernel, and if
> it is feasible/possible to make that change today.

You are upstreaming this so you have to come with rationale and detailed
analysis why existing infrastructure cannot work here.

Of course maybe it's just commit description which needs improvement.

But anyway, please do not send downstream Rpi drivers to us just because
downstream has them. That's never a good enough reason. And that is the
only impression you are making here.


...

>>> +
>>> +module_vchiq_driver(bcm2835_vcsm_cma_driver);
>>> +
>>> +MODULE_AUTHOR("Dave Stevenson");
>>> +MODULE_DESCRIPTION("VideoCore CMA Shared Memory Driver");
>>> +MODULE_LICENSE("GPL");
>>> +MODULE_ALIAS("vcsm-cma");
>>
>> Why don't you have proper ID table? How this driver is supposed to be
>> instantiated?
>>
> 
> Sorry, I was unaware of this discussion:
> https://lore.kernel.org/linux-media/2023100955-stunt-equate-c6fa@gregkh/
> 
> I will switch to using an ID table.
> 
> This and other drivers for audio, isp, codec are all instantiated as
> platform drivers under the VCHIQ bus (PATCH 13/13).


If you have a bus, not some sort of MFD device, then you should use bus
way of binding, which means in almost all cases providing an ID table,
which will generate proper aliases.

MODULE_ALIAS is basically only for the few cases where platform driver
registers another platform device or for MFD (and even then only for few
MFD cases).

Best regards,
Krzysztof