Re: [RFC PATCH v2 1/5] mtd: cfi_cmdset_0002: Add support for polling status register

[Vignesh Raghavendra <vigneshr@xxxxxx>]

On 25/03/19 10:54 PM, Joakim Tjernlund wrote:
> On Mon, 2019-03-25 at 22:36 +0530, Vignesh Raghavendra wrote:
>>
>> On 25/03/19 7:21 PM, Joakim Tjernlund wrote:
>>> On Mon, 2019-03-25 at 18:27 +0530, Vignesh Raghavendra wrote:
>>>> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you recognize the sender and know the content is safe.
>>>>
>>>>
>>>> Hi,
>>>>
>>>> On 21/03/19 11:41 PM, Joakim Tjernlund wrote:
>>>>> On Thu, 2019-03-21 at 23:15 +0530, Vignesh Raghavendra wrote:
>>>>>> HyperFlash devices are compliant with CFI AMD/Fujitsu Extended Command
>>>>>> Set(0x0002) for flash operations, therefore drivers/mtd/chips/cfi_cmdset_0002.c
>>>>>> can be use as is. But these devices do not support DQ polling method of
>>>>>> determining chip ready/good status. These flashes provide Status
>>>>>> Register whose bits can be polled to know status of flash operation.
>>>>>>
>>>>>> Cypress HyperFlash datasheet here[1], talks about CFI Amd/Fujitsu
>>>>>> Extended Query version 1.5. Bit 0 of "Software Features supported" field
>>>>>> of CFI Primary Vendor-Specific Extended Query table indicates
>>>>>> presence/absence of status register and Bit 1 indicates whether or not
>>>>>> DQ polling is supported. Using these bits, its possible to determine
>>>>>> whether flash supports DQ polling or need to use Status Register.
>>>>>>
>>>>>> Add support for polling status register to know device ready/status of
>>>>>> erase/write operations when DQ polling is not supported.
>>>>>
>>>>> Isn't this new Status scheme just a copy of Intels(cmdset_0001)?
>>>>
>>>> Yes, but with one difference: At the end of program/erase operation,
>>>> device directly enters status register mode and  starts reflecting
>>>> status register content at any address.
>>>> The device remains in the read status register state until another
>>>> command is written to the device. Therefore there is notion of device is
>>>> in "status register read mode" (FL_STATUS) state
>>>
>>> That seems to vary and long time ago RMK added this:
>>> 		/* If the flash has finished erasing, then 'erase suspend'
>>> 		 * appears to make some (28F320) flash devices switch to
>>> 		 * 'read' mode.  Make sure that we switch to 'read status'
>>> 		 * mode so we get the right data. --rmk
>>> 		 */
>>> 		map_write(map, CMD(0x70), chip->in_progress_block_addr);
>>>
>>
>> This behavior is expected with cmdset_0001. Because "The device remains
>> in the read status register state until another command is written",
>> therefore "erase suspend' command after erase completion will switch
>> device to read mode. And therefore read status is safe thing to do for
>> cmdset_0001.
>>
>> But in case of cmdset_0002 erase completion will not put device to read
>> status mode and therefore no special status tracking is required.
>>
>>>> But in case of cfi_cmdset_0002, once program/erase operation is
>>>> complete, device returns to previous address space overlay from which
>>>> operation was started from (mostly read mode)
>>>
>>> I hope you can do the same as Intel here, issue an explicit Status CMD or you will be in trouble.
>>
>> Even if we issue Read Status command to enter read status mode, any
>> single subsequent read will put device back to read mode. So, sending
>> explicit Status CMD is of not much use.
>>
>> As long as cmdset_0002 driver ensures sending Read Status cmd and next
>> single read can be done in one go (ie. mutex held), I don't see any
>> trouble here. This is already take care off.
> 
> Ouch, a non sticky Status sounds borken. Are you sure that nothing can change the
> chip between you issue the Status CMD and read out of status bits?
> Like if an erase/suspend/resume completes just after Status CMD but before Status readout?
> 

Yes, I did some tests(with HyperFlash) and erase/program
completion/suspend in b/w issue of Status CMD but before status readout
does not result in exiting status read address space overlay. So we are
safe here with non sticky Status.


-- 
Regards
Vignesh

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