linux-kernel - Re: [PATCH v9 27/28] media: iris: enable video driver probe of SM8250 SoC

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Message-ID: <9b33ba28-5aa9-4863-8fde-535841ddbc10@xs4all.nl>
Date: Wed, 8 Jan 2025 09:55:07 +0100
From: Hans Verkuil <hverkuil@...all.nl>
To: Dikshita Agarwal <quic_dikshita@...cinc.com>,
 Nicolas Dufresne <nicolas@...fresne.ca>,
 Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
 Sebastian Fricke <sebastian.fricke@...labora.com>
Cc: Vikash Garodia <quic_vgarodia@...cinc.com>,
 Abhinav Kumar <quic_abhinavk@...cinc.com>,
 Mauro Carvalho Chehab <mchehab@...nel.org>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Philipp Zabel <p.zabel@...gutronix.de>,
 Bryan O'Donoghue <bryan.odonoghue@...aro.org>,
 Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
 Neil Armstrong <neil.armstrong@...aro.org>,
 Uwe Kleine-König <u.kleine-koenig@...libre.com>,
 Jianhua Lu <lujianhua000@...il.com>,
 Stefan Schmidt <stefan.schmidt@...aro.org>, linux-media@...r.kernel.org,
 linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 27/28] media: iris: enable video driver probe of SM8250
 SoC

On 08/01/2025 09:51, Dikshita Agarwal wrote:
> 
> 
> On 1/8/2025 1:17 PM, Hans Verkuil wrote:
>> On 08/01/2025 08:43, Dikshita Agarwal wrote:
>>>
>>>
>>> On 1/7/2025 7:27 PM, Nicolas Dufresne wrote:
>>>> Le lundi 23 décembre 2024 à 16:21 +0530, Dikshita Agarwal a écrit :
>>>>>
>>>>> On 12/23/2024 4:00 PM, Mauro Carvalho Chehab wrote:
>>>>>> Em Thu, 12 Dec 2024 17:21:49 +0530
>>>>>> Dikshita Agarwal <quic_dikshita@...cinc.com> escreveu:
>>>>>>
>>>>>>> +	.dma_mask = GENMASK(31, 29) - 1,
>>>>>>
>>>>>> Setting a mask to GENMASK() - 1 sounds weird. Is it really what you want?
>>>>>> I so, why?
>>>>>>
>>>>> Hi Mauro,
>>>>>
>>>>> the value of this dma mask should be 0xe0000000 -1.
>>>>>
>>>>> The background for the same is, 0xe0000000 onward memory space is allocated
>>>>> for IO register space so we are restricting the driver buffer allocations
>>>>> to 0xe0000000 - 1.
>>>>>
>>>>> Based on the comments received in the past, we are using GENMASK to
>>>>> generate 0xe0000000.
>>>>>
>>>>> Does this answer your query or I missed something?
>>>>
>>>> I'm not sure it will do what you want. (0xe0000000 -1) matches ~BIT(29). Perhaps
>>>> you wanted to use ~0xe0000000. 
>>>>
>>> value of dma mask is coming as expected with GENMASK(31, 29) - 1
>>>
>>> qcom-iris aa00000.video-codec: dma_mask DFFFFFFF (0xe0000000 -1)
>>
>> Isn't this just the equivalent of GENMASK(28, 0)? Can't you use that?

Too early in the morning, this suggestion was clearly wrong.

>>
>> It's much easier to understand than GENMASK()-1.
> 
> Sure, I can use either ~GENMASK(29, 29) or ~BIT(29),

~BIT(29).

It's really weird to just disable a single bit, so I think some comments
explaining why this mask is needed would be good (if there aren't comments
already).

Regards,

	Hans

> Please let me know which would be better?
> 
> Thanks,
> Dikshita
> 
>>
>> Regards,
>>
>> 	Hans
>>
>>>
>>> Thanks,
>>> Dikshita
>>>> Nicolas
>>>>
>>>>>
>>>>> Thanks,
>>>>> Dikshita
>>>>>> Thanks,
>>>>>> Mauro
>>>>
>>
>>