linux-kernel - Re: Misaligned Access

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Message-ID: <CAK8P3a3MgjjspJd6S_WQZ7y+ZEPU8hRUD2QukkNQ7QxhccUn6w@mail.gmail.com>
Date:   Wed, 21 Nov 2018 21:18:03 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Vineet Gupta <vineet.gupta1@...opsys.com>
Cc:     Vitor Soares <vitor.soares@...opsys.com>,
        alexey.brodkin@...opsys.com, Joao Pinto <joao.pinto@...opsys.com>,
        jose.abreu@...opsys.com,
        "open list:SYNOPSYS ARC ARCHITECTURE" 
        <linux-snps-arc@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Misaligned Access

On Wed, Nov 21, 2018 at 9:15 PM Vineet Gupta <vineet.gupta1@...opsys.com> wrote:
>
> On 11/21/18 12:12 PM, Arnd Bergmann wrote:
> > On Wed, Nov 21, 2018 at 8:42 PM Vineet Gupta <vineet.gupta1@...opsys.com> wrote:
> >> +CC lkml, Arnd : subject matter expert
> >>
> >> On 11/21/18 10:06 AM, Vitor Soares wrote:
> >>> I use the follow function to get data from a RX Fifo.
> >>>
> >>>
> >>> static void dw_i3c_master_read_rx_fifo(struct dw_i3c_master *master,
> >>>                         u8 *bytes, int nbytes)
> >>> {
> >>>      readsl(master->regs + RX_TX_DATA_PORT, bytes, nbytes / 4);
> >> So the semantics are reading the same fifo register N times, to get the N words,
> >> hence read*s*l is appropriate. That however expects the buffer to be 4 bytes
> >> aligned, hence your issue. You can't possibly use the reads*b* as we want the
> >>
> >> The obvious but crude hack is to use a temp array for readsl and then copy over
> >> using memcpy, but I'm sure there are better ways, @Arnd ? To summarize is issue is
> >> a driver triggering unaligned access due to the misinteraction of API (driver get
> >> an unaligned u8 *) which goes against expectations of io accessor readl  (needed
> >> since the register contents are 4 bytes)
> > Is this again on ARC or some other architecture that cannot do unaligned
> > access to normal RAM? On ARMv7 or x86, you should never see a problem
> > because the CPU handles misaligned writes. On ARMv4/v5, the readsl()
> > implementation internally aligns the access to the output buffer so it
> > will work correctly.
>
> This is indeed on ARC: on ARC700 unaligned access to RAM was never supported and
> on HS38x cores, it is configurable, so the API probably needs to support both cases.

Ok, then I think you need to overwrite readsl() with a variant that
uses put_unaligned() instead the plain pointer dereference for the
output.

      Arnd