Re: [PATCH] drm/msm/adreno: De-spaghettify the use of memory barriers

[Will Deacon]

On Thu, May 16, 2024 at 01:55:26PM -0500, Andrew Halaney wrote:
> On Thu, May 16, 2024 at 08:20:05PM GMT, Akhil P Oommen wrote:
> > On Thu, May 16, 2024 at 08:15:34AM -0500, Andrew Halaney wrote:
> > > If I understand correctly, you don't need any memory barrier.
> > > writel()/readl()'s are ordered to the same endpoint. That goes for all
> > > the reordering/barrier comments mentioned below too.
> > > 
> > > device-io.rst:
> > > 
> > >     The read and write functions are defined to be ordered. That is the
> > >     compiler is not permitted to reorder the I/O sequence. When the ordering
> > >     can be compiler optimised, you can use __readb() and friends to
> > >     indicate the relaxed ordering. Use this with care.
> > > 
> > > memory-barriers.txt:
> > > 
> > >      (*) readX(), writeX():
> > > 
> > > 	    The readX() and writeX() MMIO accessors take a pointer to the
> > > 	    peripheral being accessed as an __iomem * parameter. For pointers
> > > 	    mapped with the default I/O attributes (e.g. those returned by
> > > 	    ioremap()), the ordering guarantees are as follows:
> > > 
> > > 	    1. All readX() and writeX() accesses to the same peripheral are ordered
> > > 	       with respect to each other. This ensures that MMIO register accesses
> > > 	       by the same CPU thread to a particular device will arrive in program
> > > 	       order.
> > > 
> > 
> > In arm64, a writel followed by readl translates to roughly the following
> > sequence: dmb_wmb(), __raw_writel(), __raw_readl(), dmb_rmb(). I am not
> > sure what is stopping compiler from reordering  __raw_writel() and __raw_readl()
> > above? I am assuming iomem cookie is ignored during compilation.
> 
> It seems to me that is due to some usage of volatile there in
> __raw_writel() etc, but to be honest after reading about volatile and
> some threads from gcc mailing lists, I don't have a confident answer :)
> 
> > 
> > Added Will to this thread if he can throw some light on this.
> 
> Hopefully Will can school us.

The ordering in this case is ensured by the memory attributes used for
ioremap(). When an MMIO region is mapped using Device-nGnRE attributes
(as it the case for ioremap()), the "nR" part means "no reordering", so
readX() and writeX() to that region are ordered wrt each other.

Note that guarantee _doesn't_ apply to other flavours of ioremap(), so
e.g. ioremap_wc() won't give you the ordering.

Hope that helps,

Will