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

[Lucas Stach <l.stach@pengutronix.de>]

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?

Regards,
Lucas