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

[Sui Jingfeng]

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