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

[Tien Fong Chee]

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,
>
> Best regards,
> Benoît

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.

Best regards,
Tien Fong