linux-kernel - Re: [PATCH v10 07/11] drm/etnaviv: Add support for the dma coherent device

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Message-ID: <0359ffb6-684e-777a-36ec-ad9fe4da6e48@loongson.cn>
Date:   Thu, 22 Jun 2023 00:39:13 +0800
From:   Sui Jingfeng <suijingfeng@...ngson.cn>
To:     Lucas Stach <l.stach@...gutronix.de>,
        Sui Jingfeng <18949883232@....com>,
        Russell King <linux+etnaviv@...linux.org.uk>,
        Christian Gmeiner <christian.gmeiner@...il.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        etnaviv@...ts.freedesktop.org,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH v10 07/11] drm/etnaviv: Add support for the dma coherent
 device

Hi,

On 2023/6/22 00:12, Lucas Stach wrote:
> Am Mittwoch, dem 21.06.2023 um 23:41 +0800 schrieb Sui Jingfeng:
>> On 2023/6/21 23:23, Lucas Stach wrote:
>>> Am Mittwoch, dem 21.06.2023 um 22:44 +0800 schrieb Sui Jingfeng:
>>>> Hi,
>>>>
>>>> On 2023/6/21 18:00, Lucas Stach wrote:
>>>>>> diff --git a/drivers/gpu/drm/etnaviv/etnaviv_drv.h b/drivers/gpu/drm/etnaviv/etnaviv_drv.h
>>>>>> index 9cd72948cfad..644e5712c050 100644
>>>>>> --- a/drivers/gpu/drm/etnaviv/etnaviv_drv.h
>>>>>> +++ b/drivers/gpu/drm/etnaviv/etnaviv_drv.h
>>>>>> @@ -46,6 +46,12 @@ struct etnaviv_drm_private {
>>>>>>     	struct xarray active_contexts;
>>>>>>     	u32 next_context_id;
>>>>>>     
>>>>>> +	/*
>>>>>> +	 * If true, the GPU is capable of snooping cpu cache. Here, it
>>>>>> +	 * also means that cache coherency is enforced by the hardware.
>>>>>> +	 */
>>>>>> +	bool dma_coherent;
>>>>>> +
>>>>> No need for this, I think. Just use dev_is_dma_coherent() where you
>>>>> need to know this.
>>>>>
>>>> No, we want this value cached by the driver.
>>>>
>>> Why? dev_is_dma_coherent() is a header-only function with a single
>>> pointer chasing operation. Your cache is also a single pointer chasing
>>> access, just that we now need storage for this information in both
>>> struct device and struct etnaviv_gpu.
>>
>> You don't need store it in struct etnaviv_gpu.
>>
>> As this variable is shared across the device, so it is better to be put
>> in the struct etnaviv_drm_private.
>>
>> I don't think another 4 bytes allocation is something what we can't pay for.
>>
>>
>> My patch doesn't mentioned that it need to store it inside of struct
>> etnaviv_gpu, do I?
> You are right, I was mistaken about the etnaviv struct this is added
> to. However there is still the fundamental question: what's the gain of
> this cache? The information is already available in struct device and
> will be accessed with the same amount of loads if you care that much
> about micro-optimization.

Sometime, in some function it is more convenient(easier) to fetch 
'struct etnaviv_drm_private *priv'

than the 'struct device *dev',  I think this is obvious.

> Regards,
> Lucas

-- 
Jingfeng