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

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Date:   Wed, 21 Jun 2023 23:41:21 +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


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?

> Regards,
> Lucas
>
>> We only need call  dev_is_dma_coherent() once!
>>
>> We need to reuse this variable on other places.
>>
-- 
Jingfeng