linux-kernel - Re: [PATCH] mtd: phram: Map RAM using memremap instead of ioremap

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Date:   Tue, 24 May 2022 09:13:41 +0200
From:   Petr Malat <oss@...at.biz>
To:     David Laight <David.Laight@...lab.com>
Cc:     "linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
        Joern Engel <joern@...ybastard.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH] mtd: phram: Map RAM using memremap instead of ioremap

On Mon, May 23, 2022 at 04:09:20PM +0000, David Laight wrote:
> From: Petr Malat
> > Sent: 23 May 2022 16:28
> > On Mon, May 23, 2022 at 02:51:41PM +0000, David Laight wrote:
> > > From: Petr Malat
> > > > Sent: 23 May 2022 15:28
> > > >
> > > > One can't use memcpy on memory obtained by ioremap, because IO memory
> > > > may have different alignment and size access restriction than the system
> > > > memory. Use memremap as phram driver operates on RAM.
> > >
> > > Does that actually help?
> > > The memcpy() is still likely to issue unaligned accesses
> > > that the hardware can't handle.
> > 
> > Yes, it solves the issue. Memcpy can cause unaligned access only on
> > platforms, which can handle it. And on ARM64 it's handled only for
> > RAM and not for a device memory (__pgprot(PROT_DEVICE_*)).
> 
> Does mapping it as memory cause it to be cached?
> So the hardware only sees cache line reads (which are aligned)
> and the cpu support for misaligned memory accesses then
> stop the faults?
Yes, this is controlled by the MEMREMAP_WB flag, which sets up
a mapping, which "matches the default mapping for System RAM on
the architecture. This is usually a read-allocate write-back cache.

> On x86 (which I know a lot more about) memcpy() has a nasty
> habit of getting implemented as 'rep movsb' relying on the
> cpu to speed it up.
> But that doesn't happen for uncached addresses - so you get
> very slow byte copies.
> OTOH misaligned PCIe transfers generate TLP that have the
> correct byte enables for the end words.
> Provided the PCIe target isn't broken they are fine.
With memremap one should get the same behavior and performance
as with the system memory and it seems to be a good choice for
"Physical system RAM" MTD driver, but if one uses it for actual
IO memory, he should use ioremam, memcpy_toio and memcpy_fromio.
Using these prevents the crash on arm64 as well, but could lead
to a performance degradation on some platforms.

If you think there could be users using the driver for a real IO
mem, I can provide both behaviors and let the user choose with an
option.
  Petr