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Date:   Wed, 24 Feb 2021 22:00:05 +0530
From:   mdalam@codeaurora.org
To:     Miquel Raynal <miquel.raynal@bootlin.com>
Cc:     mani@kernel.org, boris.brezillon@collabora.com,
        linux-mtd@lists.infradead.org, linux-kernel@vger.kernel.org,
        sricharan@codeaurora.org, mdalam=codeaurora.org@codeaurora.org
Subject: Re: [PATCH] mtd: rawnand: qcom: update last code word register
In-Reply-To: <20210224074850.0c74154d@xps13>
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On 2021-02-24 12:18, Miquel Raynal wrote:
> Hello,
> 
> mdalam@codeaurora.org wrote on Wed, 24 Feb 2021 10:09:48 +0530:
> 
>> On 2021-02-24 01:13, mdalam@codeaurora.org wrote:
>> > On 2021-02-23 22:04, Miquel Raynal wrote:
>> >> Hello,
>> >> >> Md Sadre Alam <mdalam@codeaurora.org> wrote on Tue, 23 Feb 2021
>> >> 01:34:27 +0530:
>> >> >>> From QPIC version 2.0 onwards new register got added to read last
>> >> >>                                a new
>> >> >>> codeword. This change will add the READ_LOCATION_LAST_CW_n register.
>> >> >>             Add support for this READ_LOCATION_LAST_CW_n register.
>> >> >>> >>> For first three code word READ_LOCATION_n register will be
>> >>> use.For last code word READ_LOCATION_LAST_CW_n register will be
>> >>> use.
>> >> >> "
>> >> In the case of QPIC v2, codewords 0, 1 and 2 will be accessed through
>> >> READ_LOCATION_n, while codeword 3 will be accessed through
>> >> READ_LOCATION_LAST_CW_n.
>> >> "
>> >> >> When I read my own sentence, I feel that there is something wrong.
>> >> If there are only 4 codewords, I guess a QPIC v2 is able to use
>> >> READ_LOCATION_3 or READ_LOCATION_LAST_CW_0 interchangeably. Isn't it?
>> >> >> I guess the point of having these "last_cw_n" registers is to support
>> >> up to 8 codewords, am I wrong? If this the case, the current patch
>> >> completely fails doing that I don't get the point of such change.
>> >
>> > This register is only use to read last code word.
>> >
>> > I have address all the comments from all the previous sub sequent
>> > patches and pushed
>> > all patches in only one series.
>> >
>> > Please check.
>> 
>>   The registers READ_LOCATION & READ_LOCATION_LAST are not associated 
>> with number of code words.
>>   These two registers are used to access the location inside a code 
>> word.
> 
> Ok. Can you please explain what is a location then? Or point me to a
> datasheet that explains it.

   The location is the position inside a code word.

> 
> Bottom line question: why having READ_LOCATION_0, _1,... an
> READ_LOCATION_LAST_0, _1, etc?

  READ_LOCATION_0, _1,... are used to extract multiple chunks from a code 
word.

  e.g If we wanted to extract first 100 bytes from a code word then 
(0...99) READ_LOCATION_0 will be configured.
      if we wanted to extract next 100 bytes (100...199) then 
READ_LOCATION_1 will be configured.

      same way for last code word READ_LOCATION_LAST_0, _1, will be used.


   Below is the mapping example for a 2K page size.

  
<---------------------2048------------------------------------------------><------64---------->2111
  
_______________________________________________________________________________________________
|                                                                        
   |   |               |
|                                                                        
   |   |               |
|                                                                        
   |   |               |
|                                                                        
   |   |               |
|__________________________________________________________________________|___|_______________|
                                                                          
       2048-bytes
                                                                          
       factory BBM


One code word:

              1-byte BBM       7-bytes ECC
______________________________________
|            |  |            |   |    |
|            |  |            |   |    |
|            |  |            |   |    |
|            |  |            |   |    |
|            |  |            |   |    |
|____________|__|____________|___|____|
  <-----------512 bytes------->    4-bytes spare
              data


<-------------------------------------2048-------------------------------------------------->2047<------64---------->2111
  
____________________________________________________________________________________________________________________
|                                                                        
                      |   |             |    |
|                                                                        
                      |   |             |    |
|                                                                        
                      |   |             |    |
|                                                                        
                      |   |             |    |
|_____________________________________________________________________________________________|___|_____________|____|
                                                                          
                             |   |
                                                                          
                             |   |
                                                                          
                             |   |<-----mapping
                                                                          
                             |   |
                                                                          
                             |   |
  
____________________________________________________________________________________________________|___|______
|                      |                           |                     
           |                            |
|                      |                           |                     
           |                            |
|                      |                           |                     
           |                            |
|                      |                           |                     
           |                            |
|                      |                           |                     
           |                            |
|______________________|___________________________|_______________________________|____________________________|
<-------CW1-----------><----------CW2-------------><-----------CW3----------------><----------CW4--------------->
                      /                             \
                     /                                \
                   /                                     \
                 /                                         \
               /                                             \
              /                                                 \
            /                                                      \
          /                                                            \
         
_________________________________________________________________ \___
        |                                                                 
     |
        |                                                                 
     |
        |                                                                 
     |
        |                                                                 
     |
        |                                                                 
     |
        |                                                                 
     |
        |<--read_location_0-->   <---read_location_1-->  
<--read_location_n-->|                                                   
               |
        
|_____________________________________________________________________|


So in QPIC V2 onwards to access last code word same way we have to use 
read_location_last_0,1,2..3 etc.


> 
>> So whether we are having 4 code words
>>   or 8 code words it doesn't matter. If we wanted access the location 
>> within normal code word we have to
>>   use READ_LOCATION register and if we wanted to access location in 
>> last code word then we have to use
>>   READ_LOCATION_LAST.
>> >
>> >> >>> Signed-off-by: Md Sadre Alam <mdalam@codeaurora.org>
> 
> Thanks,
> Miquèl