linux-kernel - Re: [PATCH RFC v4 4/6] drm/sprd: add Unisoc's drm display controller driver

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Message-ID: <20200307161445.GA7524@ravnborg.org>
Date:   Sat, 7 Mar 2020 17:14:45 +0100
From:   Sam Ravnborg <sam@...nborg.org>
To:     tang pengchuan <kevin3.tang@...il.com>
Cc:     Emil Velikov <emil.l.velikov@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        Baolin Wang <baolin.wang@...aro.org>,
        Dave Airlie <airlied@...ux.ie>,
        Chunyan Zhang <zhang.lyra@...il.com>,
        "Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        ML dri-devel <dri-devel@...ts.freedesktop.org>,
        Orson Zhai <orsonzhai@...il.com>, Sean Paul <sean@...rly.run>
Subject: Re: [PATCH RFC v4 4/6] drm/sprd: add Unisoc's drm display controller
 driver

Hi Kevin

> > > +
> > > +ifdef CONFIG_ARM64
> > > +KBUILD_CFLAGS += -mstrict-align
> >
> >
> > There are many other drivers that do not use readl/writel for register access,
> > yet none has this workaround... Even those that they are exclusively ARM64.
> >
> > Have you tried that it's not a buggy version of GCC? At the very least, I'd
> > encourage you to add a brief comment about the problem + setup.
> >
> > ... In general I think one should follow the suggestions from Rob Herring.
> >
> Yocto v2.5
> aarch64-linaro-linux-gcc (Linaro GCC 7.2-2017.11) 7.2.1 20171011
> 
> Crash Stack:
> /sprd/drv/dispc/dpu_r2p0.c:729
> 1796256 ffffff8008486650:       f803c043        stur    x3, [x2,#60]
> =>Unhandled fault: alignment fault (0x96000061) at 0xffffff80098b883c
> 
> 729         reg->mmu_min_ppn1 = 0;
> 730         reg->mmu_ppn_range1 = 0xffff;
> 731         reg->mmu_min_ppn2 = 0;
> 732         reg->mmu_ppn_range2 = 0xffff;
> 
> The above C code operation are continuous. The compiler may think that
> the access can be completed by directly using two 64-bit assignment
> operations, so it is optimized to 64-bit operation.

What you see is a side-effect of using a sturct for register access.
When you ave your code change to use readl()/writel() and friends
this is no logner a problem, and you can drop the cc flag.

	Sam