linux-kernel - Re: [PATCH 6/7] staging: fsl-mc: rewrite mc command send/receive to work on 32-bits

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Date:   Mon, 17 Jul 2017 14:27:57 +0000
From:   Laurentiu Tudor <laurentiu.tudor@....com>
To:     Arnd Bergmann <arnd@...db.de>
CC:     gregkh <gregkh@...uxfoundation.org>,
        Stuart Yoder <stuyoder@...il.com>,
        "devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Alexander Graf <agraf@...e.de>,
        Ioana Ciornei <ioana.ciornei@....com>,
        Ruxandra Ioana Radulescu <ruxandra.radulescu@....com>,
        Bharat Bhushan <bharat.bhushan@....com>,
        Catalin Horghidan <catalin.horghidan@....com>,
        Leo Li <leoyang.li@....com>, Roy Pledge <roy.pledge@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 6/7] staging: fsl-mc: rewrite mc command send/receive to
 work on 32-bits

Hi Arnd,

On 07/17/2017 04:45 PM, Arnd Bergmann wrote:
> On Mon, Jul 17, 2017 at 3:26 PM,  <laurentiu.tudor@....com> wrote:
>> From: Laurentiu Tudor <laurentiu.tudor@....com>
>>
>> Split the 64-bit accesses in 32-bit accesses because there's no real
>> constrain in MC to do only atomic 64-bit. There's only one place
>> where ordering is important: when writing the MC command header the
>> first 32-bit part of the header must be written last.
>> We do this switch in order to allow compiling the driver on 32-bit.
>>
>> Signed-off-by: Laurentiu Tudor <laurentiu.tudor@....com>
>> ---
>>   drivers/staging/fsl-mc/bus/mc-sys.c | 31 ++++++++++++-------------------
>>   1 file changed, 12 insertions(+), 19 deletions(-)
>>
>> diff --git a/drivers/staging/fsl-mc/bus/mc-sys.c b/drivers/staging/fsl-mc/bus/mc-sys.c
>> index 195d9f3..dd2828e 100644
>> --- a/drivers/staging/fsl-mc/bus/mc-sys.c
>> +++ b/drivers/staging/fsl-mc/bus/mc-sys.c
>> @@ -124,14 +124,15 @@ static inline void mc_write_command(struct mc_command __iomem *portal,
>>   {
>>          int i;
>>
>> -       /* copy command parameters into the portal */
>> -       for (i = 0; i < MC_CMD_NUM_OF_PARAMS; i++)
>> -               __raw_writeq(cmd->params[i], &portal->params[i]);
>> -       /* ensure command params are committed before submitting it */
>> -       wmb();
>> -
>> -       /* submit the command by writing the header */
>> -       __raw_writeq(cmd->header, &portal->header);
>> +       /*
>> +        * copy command parameters into the portal. Final write
>> +        * triggers the submission of the command.
>> +        */
>> +       for (i = sizeof(struct mc_command) / sizeof(u32) - 1; i >= 0; i--) {
>> +               __raw_writel(((u32 *)cmd)[i], &((u32 *)portal)[i]);
>> +               /* ensure command params are committed before submitting it */
>> +               wmb();
>> +       }
>>   }
>
> What is the byte order requirement on this buffer?

Endianness is handled by the callers so this function must leave
the binary blob intact.

> If this is a byte string
> rather than individual registers, you should probably just use
> memcpy_toio()

It's a header followed by an opaque command. The protocol for queueing a 
command says that the first 32-bit half of the header must be written 
last, this triggering the command handling in the MC.

> but if these are separate registers, then using the
> __raw_* accessors is still wrong, at least on kernels that have a
> different byteorder from the hardware.

As mentioned above, endianness is handled by the caller. This function
takes raw data and must leave it unchanged.

> Also, are you sure that adding those six extra barriers has no
> performance impact?

This is a slow interface used in slow paths, so i don't think those 
extra barriers will have any performance impact.

---
Thanks & Best Regards, Laurentiu