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 23:00:39 +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 18:00, Lucas Stach wrote:
>>   static inline enum dma_data_direction etnaviv_op_to_dma_dir(u32 op)
>> @@ -369,6 +381,7 @@ int etnaviv_gem_cpu_prep(struct drm_gem_object *obj, u32 op,
>>   {
>>   	struct etnaviv_gem_object *etnaviv_obj = to_etnaviv_bo(obj);
>>   	struct drm_device *dev = obj->dev;
>> +	struct etnaviv_drm_private *priv = dev->dev_private;
>>   	bool write = !!(op & ETNA_PREP_WRITE);
>>   	int ret;
>>   
>> @@ -395,7 +408,7 @@ int etnaviv_gem_cpu_prep(struct drm_gem_object *obj, u32 op,
>>   			return ret == 0 ? -ETIMEDOUT : ret;
>>   	}
>>   
>> -	if (etnaviv_obj->flags & ETNA_BO_CACHED) {
>> +	if (!priv->dma_coherent && etnaviv_obj->flags & ETNA_BO_CACHED) {
> Why do you need this? Isn't dma_sync_sgtable_for_cpu a no-op on your
> platform when the device is coherent?
>
I need this to show that our hardware is truly dma-coherent!

I have tested that the driver still works like a charm without adding 
this code '!priv->dma_coherent'.


But I'm expressing the idea that a truly dma-coherent just device don't 
need this.

I don't care if it is a no-op.

It is now, it may not in the future.

Even it is, the overhead of function call itself still get involved.

Also, we want to try flush the write buffer with the CPU manually.


Currently, we want the absolute correctness in the concept,

not only the rendering results.

-- 
Jingfeng