linux-kernel - Re: [PATCH v6 5/6] drm/etnaviv: add driver support for the PCI devices

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Message-ID: <6668b162-5de1-8b2e-06d3-577f098e6ed4@loongson.cn>
Date:   Thu, 1 Jun 2023 00:32:55 +0800
From:   Sui Jingfeng <suijingfeng@...ngson.cn>
To:     Lucas Stach <l.stach@...gutronix.de>,
        Bjorn Helgaas <helgaas@...nel.org>
Cc:     Russell King <linux+etnaviv@...linux.org.uk>,
        Christian Gmeiner <christian.gmeiner@...il.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>, linux-kernel@...r.kernel.org,
        etnaviv@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        loongson-kernel@...ts.loongnix.cn, Li Yi <liyi@...ngson.cn>
Subject: Re: [PATCH v6 5/6] drm/etnaviv: add driver support for the PCI
 devices

Hi,

On 2023/6/1 00:23, Lucas Stach wrote:
> Hi Sui Jingfeng,
>
> Am Donnerstag, dem 01.06.2023 um 00:08 +0800 schrieb Sui Jingfeng:
>> Hi,
>>
>> On 2023/5/31 03:02, Bjorn Helgaas wrote:
>>> On Wed, May 31, 2023 at 12:06:42AM +0800, Sui Jingfeng wrote:
>>>> This patch adds PCI driver support on top of what already have. Take the
>>>> GC1000 in LS7A1000/LS2K1000 as the first instance of the PCI device driver.
>>>> There is only one GPU core for the GC1000 in the LS7A1000 and LS2K1000.
>>>> Therefore, component frameworks can be avoided. Because we want to bind the
>>>> DRM driver service to the PCI driver manually.
>>>> +	 * Loongson Mips and LoongArch CPU(ls3a5000, ls3a4000, ls2k1000la)
>>>> +	 * maintain cache coherency by hardware
>>>> +	 */
>>>> +	if (IS_ENABLED(CONFIG_CPU_LOONGSON64) || IS_ENABLED(CONFIG_LOONGARCH))
>>>> +		priv->has_cached_coherent = true;
>>> This looks like something that should be a runtime check, not a
>>> compile-time check.
>>>
>>> If it's possible to build a single kernel image that runs on Loongson
>>> MIPS or LoongArch CPU and, in addition, runs on other platforms, you
>>> cannot assume that all the others maintain this cache coherency.
>> Nice catch! I don't even realize this!
>>
>>
>> LS3A4000 is mips64r2 with MSA SIMD, while LS3A5000 is LoongArch,
>>
>> instruction set, compiler, and binary interface are totally changed.
>>
>> Therefore, it's impossible to build a single kernel image that runs on
>> all Loongson CPUs.
>>
>> Currently, I can guarantee that this works on the Loongson platform.
>>
>> My initial intent here is to let priv->has_cached_coherent be *true* on
>> the Loongson platform (both mips and loongarch).
>>
>> I do know there are some other vendors who bought GPU IP from Vivante.
>>
>> say GC7000, and integrate it into their discrete GPU product.
>>
>> But it is also a PCI device, but this is another story; it deserves
>> another patch.
>>
>> I don't know if Etnaviv folk find some similar hardware on Arm Arch,
>>
>> Some Arm CPUs do not maintain cached coherency on hardware.
>>
>> The has_cached_coherent member can be set to false on such hardware.
>>
>> For us, it seems that there is no need to do runtime checking,
>>
>> because they are all cached coherent by default.
>>
>>
>> Can I improve this in the future, currently I don't have a good idea.
> I think I mentioned before that this needs to be a runtime check. What
> does dev_is_dma_coherent() return for the Vivante GPU device on your
> platform?

Yes, you have told me so.

I will try it, and I will answer your question tomorrow.

> Regards,
> Lucas

-- 
Jingfeng