linux-kernel - Re: [PATCH v2 6/8] staging: fsl-mc: don't use raw device io functions

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Message-ID: <596E1A77.4030204@nxp.com>
Date:   Tue, 18 Jul 2017 14:26:00 +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>,
        Robin Murphy <robin.murphy@....com>,
        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 v2 6/8] staging: fsl-mc: don't use raw device io functions

Hi Arnd,

On 07/18/2017 05:18 PM, Arnd Bergmann wrote:
> On Tue, Jul 18, 2017 at 3:37 PM,  <laurentiu.tudor@....com> wrote:
>> From: Laurentiu Tudor <laurentiu.tudor@....com>
>>
>> As raw device io functions are not portable and don't handle byte-order
>> (triggering suspicion that endianness isn't handled well) switch to
>> using the standard api.
>> Since MC expects LE byte-order and the upper layers already take care
>> of that, we need to trick the device io api by doing a LE -> CPU
>> conversion just before calling it. This way, the CPU -> LE conversion
>> done in the api puts the data back in the right byte-order. Obviously,
>> for reads the extra step is mirrored: there's a CPU -> LE conversion
>> following the API call.
>>
>> Signed-off-by: Laurentiu Tudor <laurentiu.tudor@....com>
>> ---
>> Notes:
>>      -v2
>>        -new patch replacing https://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flkml.org%2Flkml%2F2017%2F7%2F17%2F419&data=01%7C01%7Claurentiu.tudor%40nxp.com%7C77381272b4914c9ac64708d4cde7d94e%7C686ea1d3bc2b4c6fa92cd99c5c301635%7C0&sdata=FTVLKox6T4i9OFmb%2B5BkSEDQrDrafXznY6nsJ0dgFSk%3D&reserved=0
>>
>>   drivers/staging/fsl-mc/bus/mc-sys.c | 21 +++++++++++++++------
>>   1 file changed, 15 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/staging/fsl-mc/bus/mc-sys.c b/drivers/staging/fsl-mc/bus/mc-sys.c
>> index 195d9f3..8a6dc47 100644
>> --- a/drivers/staging/fsl-mc/bus/mc-sys.c
>> +++ b/drivers/staging/fsl-mc/bus/mc-sys.c
>> @@ -126,12 +126,15 @@ static inline void mc_write_command(struct mc_command __iomem *portal,
>>
>>          /* 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();
>> +               /*
>> +                * Data is already in the expected LE byte-order. Do an
>> +                * extra LE -> CPU conversion so that the CPU -> LE done in
>> +                * the device io write api puts it back in the right order.
>> +                */
>> +               writeq_relaxed(le64_to_cpu(cmd->params[i]), &portal->params[i]);
>>
>>          /* submit the command by writing the header */
>> -       __raw_writeq(cmd->header, &portal->header);
>> +       writeq(le64_to_cpu(cmd->header), &portal->header);
>>   }
>
> Looks good, but just to be sure this is what you intended:
>
> On 32-bit systems, this will now write val>>32 to cmd->header+4,
> followed by writing val&0xffffffff to cmd->header.

Right. That's how it should happen.

> You said earlier that the command is triggered when the final
> four bytes are written, but it looks like the order is wrong now.
>
> Should you use io-64-nonatomic-lo-hi.h instead of
> io-64-nonatomic-hi-lo.h then?
>

Maybe i made an error in my previous emails, but the hi-lo variant is
the correct one. The command execution is triggered when the _first_ 
32-bit half of the header (header&0xffffffff) is written, so that's why 
it must be written last.

---
Thanks & Best Regards, Laurentiu