linux-kernel - Re: Misaligned Access

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Message-ID: <C2D7FE5348E1B147BCA15975FBA23075014641664A@US01WEMBX2.internal.synopsys.com>
Date:   Wed, 21 Nov 2018 20:15:54 +0000
From:   Vineet Gupta <vineet.gupta1@...opsys.com>
To:     Arnd Bergmann <arnd@...db.de>
CC:     Vitor Soares <vitor.soares@...opsys.com>,
        "alexey.brodkin@...opsys.com" <alexey.brodkin@...opsys.com>,
        Joao Pinto <joao.pinto@...opsys.com>,
        "jose.abreu@...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 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.

Thx,
-Vineet