linux-kernel - Re: [PATCH] mtd: core: handle flashes without OTP gracefully

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Message-ID: <d35a1f0d-5a26-82d5-c051-3d9094449602@roeck-us.net>
Date:   Tue, 6 Jul 2021 11:22:37 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Miquel Raynal <miquel.raynal@...tlin.com>
Cc:     Tudor.Ambarus@...rochip.com, michael@...le.cc,
        linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org,
        richard@....at, vigneshr@...com
Subject: Re: [PATCH] mtd: core: handle flashes without OTP gracefully

On 7/6/21 9:29 AM, Miquel Raynal wrote:
> Hi Guenter,
> 
> Guenter Roeck <linux@...ck-us.net> wrote on Sat, 3 Jul 2021 10:26:06
> -0700:
> 
>> On 7/3/21 9:42 AM, Tudor.Ambarus@...rochip.com wrote:
>>> On 7/3/21 7:08 PM, Michael Walle wrote:
>>>> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
>>>>
>>>> Am 3. Juli 2021 11:56:14 MESZ schrieb Tudor.Ambarus@...rochip.com:
>>>>> On 7/2/21 12:38 PM, Michael Walle wrote:
>>>>>> EXTERNAL EMAIL: Do not click links or open attachments unless you
>>>>> know the content is safe
>>>>>>
>>>>>> There are flash drivers which registers the OTP callbacks although
>>>>> the
>>>>>> flash doesn't support OTP regions and return -ENODATA for these
>>>>>> callbacks if there is no OTP. If this happens, the probe of the whole
>>>>>
>>>>> why do they register the OTP callback if they don't support OTP?
>>>>
>>>> I don't know. But I certainly won't touch that code :p
>>>
>>> why? :D
>>>    
>>>>
>>>>   
>>>>>> flash will fail. Fix it by handling the ENODATA return code and skip
>>>>>> the OTP region nvmem setup.
>>>>>>
>>>>>> Fixes: 4b361cfa8624 ("mtd: core: add OTP nvmem provider support")
>>>>>> Reported-by: Guenter Roeck <linux@...ck-us.net>
>>>>>> Signed-off-by: Michael Walle <michael@...le.cc>
>>>>>> ---
>>>>>>    drivers/mtd/mtdcore.c | 10 ++++++++--
>>>>>>    1 file changed, 8 insertions(+), 2 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/mtd/mtdcore.c b/drivers/mtd/mtdcore.c
>>>>>> index b5ccd3037788..6881d1423dd6 100644
>>>>>> --- a/drivers/mtd/mtdcore.c
>>>>>> +++ b/drivers/mtd/mtdcore.c
>>>>>> @@ -880,7 +880,10 @@ static int mtd_otp_nvmem_add(struct mtd_info
>>>>> *mtd)
>>>>>>
>>>>>>           if (mtd->_get_user_prot_info && mtd->_read_user_prot_reg) {
>>>>>>                   size = mtd_otp_size(mtd, true);
>>>>>> -               if (size < 0)
>>>>>> +               /* ENODATA means there is no OTP region */
>>>>>> +               if (size == -ENODATA)
>>>>>
>>>>> If no OTP data, maybe it's more appropriate for the clients to just
>>>>> return a retlen of 0.
>>>>
>>>> you mean already checking ENODATA in mtd_otp_size() and return 0. That would also make the hunk below unnecessary. I'll change it.
>>>
>>> I've thought about:
>>>
>>> diff --git a/drivers/mtd/chips/cfi_cmdset_0001.c b/drivers/mtd/chips/cfi_cmdset_0001.c
>>> index 54f92d09d9cf..9419b33d7238 100644
>>> --- a/drivers/mtd/chips/cfi_cmdset_0001.c
>>> +++ b/drivers/mtd/chips/cfi_cmdset_0001.c
>>> @@ -2314,7 +2314,7 @@ static int cfi_intelext_otp_walk(struct mtd_info *mtd, loff_t from, size_t len,
>>>    >          /* Check that we actually have some OTP registers */
>>>           if (!extp || !(extp->FeatureSupport & 64) || !extp->NumProtectionFields)
>>> -               return -ENODATA;
>>> +               return 0;
>>>    
>>
>> There are various places where this is called, including code returning information
>> to userspace. That means you'd be changing the ABI to userspace which would now suddenly
>> return 0 instead of -ENODATA.
> 
> Yeah let's avoid this if possible, even though I liked Tudor's approach.
> 
> Would Michael proposal of checking it in mtd_otp_size() still affect
> userspace? If not, having a single check over the -ENODATA return code
> seems attractive.
> 

The check in mtd_otp_nvmem_add() does not affect userspace.

Guenter