[PATCH V2] mtd: bcm47xxsflash: support reading flash out of mapping window

[Rafał Miłecki]

On 17 January 2017 at 12:49, Marek Vasut <marek.vasut at gmail.com> wrote:
> On 01/17/2017 12:08 PM, Rafał Miłecki wrote:
>> On 17 January 2017 at 12:00, Marek Vasut <marek.vasut at gmail.com> wrote:
>>> On 01/17/2017 11:51 AM, Rafał Miłecki wrote:
>>>> From: Rafał Miłecki <rafal at milecki.pl>
>>>>
>>>> For reading flash content we use MMIO but it's possible to read only
>>>> first 16 MiB this way. It's simply an arch design/limitation.
>>>> To support flash sizes bigger than 16 MiB implement indirect access
>>>> using ChipCommon registers.
>>>> This has been tested using MX25L25635F.
>>>>
>>>> Signed-off-by: Rafał Miłecki <rafal at milecki.pl>
>>>> ---
>>>> V2: Simplify line writing to buf
>>>>     Add some trivial comment for OPCODE_ST_READ4B
>>>>     Both requested by Marek
>>>> ---
>>>>  drivers/mtd/devices/bcm47xxsflash.c | 12 +++++++++++-
>>>>  drivers/mtd/devices/bcm47xxsflash.h |  3 +++
>>>>  2 files changed, 14 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/mtd/devices/bcm47xxsflash.c b/drivers/mtd/devices/bcm47xxsflash.c
>>>> index 4decd8c..8d4c1db 100644
>>>> --- a/drivers/mtd/devices/bcm47xxsflash.c
>>>> +++ b/drivers/mtd/devices/bcm47xxsflash.c
>>>> @@ -110,7 +110,17 @@ static int bcm47xxsflash_read(struct mtd_info *mtd, loff_t from, size_t len,
>>>>       if ((from + len) > mtd->size)
>>>>               return -EINVAL;
>>>>
>>>> -     memcpy_fromio(buf, b47s->window + from, len);
>>>> +     if (from + len <= BCM47XXSFLASH_WINDOW_SIZE) {
>>>> +             memcpy_fromio(buf, b47s->window + from, len);
>>>
>>> One last nit, what if the read starts < 16 MiB and ends > 16 MiB ?
>>> Shouldn't that use partly the windowed mode and partly the other mode?
>>
>> You'll lost 10ns*... or not as splitting it into 2 code paths could
>> take longer, who knows. Most access are block aligned anyway. I don't
>> think such corner case with doubtful gain is much worth considering &
>> optimizing.
>
> So you can also read the first 16 MiB using the new method , right ?

I could, but this could be noticeable in performance. Reading 16 MiB
using slower method is different from reading what... a few KiB? Are
you actually sure mtd does unaligned reads at all?

Please let's argue about real problems and not few % performance in
some corner case on some obsolete hardware with poor design.

-- 
Rafał