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

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Message-ID: <87c9576e6ca1b58fa94e0bc1a2f4be3847f0518c.camel@pengutronix.de>
Date:   Wed, 21 Jun 2023 17:23:21 +0200
From:   Lucas Stach <l.stach@...gutronix.de>
To:     Sui Jingfeng <suijingfeng@...ngson.cn>,
        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

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.

Regards,
Lucas

> We only need call  dev_is_dma_coherent() once!
> 
> We need to reuse this variable on other places.
>