[U-Boot] [PATCH 2/2] fs/fat/fat: Optimizes memory size with single global variable instead of 3

[Stephen Warren]

On 07/18/2016 09:53 PM, Tien Fong Chee wrote:
> On Fri, 2016-07-15 at 01:37 +0200, Benoît Thébaudeau wrote:
>> Dear Tien Fong,
>>
>> On Thu, Jul 14, 2016 at 12:48 PM, Tien Fong Chee <tfchee at altera.com>
>> wrote:
>>> Dear Benoît,
>>>
>>> On Wed, 2016-07-13 at 12:56 +0200, Benoît Thébaudeau wrote:
>>>> Dear Tien Fong Chee,
>>>>
>>>> On Jul 13, 2016 at 11:01 AM, Tien Fong Chee wrote:
>>>>> Single 64KB get_contents_vfatname_block global variable would
>>>>> be
>>>>> used for
>>>>> all FAT implementation instead of allocating additional two
>>>>> global
>>>>> variables
>>>>> which are get_denfromdir_block and do_fat_read_at_block. This
>>>>> implementation
>>>>> can help in saving up 128KB memory space.
>>>>>
>>>>> Signed-off-by: Tien Fong Chee <tfchee at altera.com>
>>>>> Cc: Dinh Nguyen <dinguyen at opensource.altera.com>
>>>>> Cc: Dinh Nguyen <dinh.linux at gmail.com>
>>>>> Cc: ChinLiang <clsee at altera.com>
>>>>> Cc: Vagrant Cascadian <vagrant at debian.org>
>>>>> Cc: Simon Glass <sjg at chromium.org>
>>>>> Cc: Stephen Warren <swarren at nvidia.com>
>>>>> Cc: Benoît Thébaudeau <benoit at wsystem.com>
>>>>> ---
>>>>>   fs/fat/fat.c |    6 ++----
>>>>>   1 files changed, 2 insertions(+), 4 deletions(-)
>>>>>
>>>>> diff --git a/fs/fat/fat.c b/fs/fat/fat.c
>>>>> index 826bd85..5d1afe6 100644
>>>>> --- a/fs/fat/fat.c
>>>>> +++ b/fs/fat/fat.c
>>>>> @@ -579,8 +579,7 @@ static __u8 mkcksum(const char name[8],
>>>>> const
>>>>> char ext[3])
>>>>>    * Get the directory entry associated with 'filename' from the
>>>>> directory
>>>>>    * starting at 'startsect'
>>>>>    */
>>>>> -__u8 get_dentfromdir_block[MAX_CLUSTSIZE]
>>>>> -   __aligned(ARCH_DMA_MINALIGN);
>>>>> +__u8 *get_dentfromdir_block = get_contents_vfatname_block;
>>>>>
>>>>>   static dir_entry *get_dentfromdir(fsdata *mydata, int
>>>>> startsect,
>>>>>                                char *filename, dir_entry
>>>>> *retdent,
>>>>> @@ -811,8 +810,7 @@ exit:
>>>>>      return ret;
>>>>>   }
>>>>>
>>>>> -__u8 do_fat_read_at_block[MAX_CLUSTSIZE]
>>>>> -   __aligned(ARCH_DMA_MINALIGN);
>>>>> +__u8 *do_fat_read_at_block = get_contents_vfatname_block;
>>>>>
>>>>>   int do_fat_read_at(const char *filename, loff_t pos, void
>>>>> *buffer,
>>>>>                 loff_t maxsize, int dols, int dogetsize, loff_t
>>>>> *size)
>>>>
>>>> This probably breaks at least fat_write.c, which uses:
>>>>    memcpy(get_dentfromdir_block, get_contents_vfatname_block,
>>>
>>> With this patch, this line code "memcpy(get_dentfromdir_block,
>>> get_contents_vfatname_block," is not required anymore since both
>>>   get_dentfromdir_block and get_contents_vfatname_block are sharing
>>> the
>>> same content and memory. So, this line code can be removed or
>>> leaving
>>> in there. However, there is only one place within
>>> fill_dir_slot_buffer
>>> function where it can corrupt the the global memory, and it is
>>> fixed by
>>> replacing with local buffer. This was sent out with another patch
>>> called "[PATCH 1/2] fs/fat/fatwrite: Local variable as buffer to
>>> store
>>> dir_slot entries". Overall, applying these two patches, a lot
>>> memory
>>> space can be saved and fitting into small OCRAM, but need to be
>>> very
>>> careful when modifying the code related to global memory.
>>
>> I get the point, but I am a bit concerned because these changes make
>> the code even more fragile and hard to maintain than it currently is.
> Yeah, i agree with you, and there is trade-off in saving the memory
> space.
>> Perhaps it would be time to switch to FatFs as previously suggested.
>> Here is its memory usage:
>> http://elm-chan.org/fsw/ff/en/appnote.html#memory
>>
> Cool. If i am not mistaken, this implementation would impact a lot of
> areas, especially interface level. What's about the testing? I am still
> voting for this simple patch changes, until we have enough resources to
> do the switching.

I think switching to the FF library is a non-starter. It's 
excruciatingly slow since it always reads even contiguous files one 
cluster at a time. I did propose a change to the library, but the 
maintainer didn't seem interested in fixing the problem. If we were to 
switch, the Tianocore implementation might be worth looking at, now 
they've fixed the license of the FAT code to remove the requirement that 
it only be used in EFI implementations. I haven't looked the code to 
know whether it'd be possible/good to switch though.