From patchwork Fri Nov 7 21:37:57 2008 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Subject: clarify and improve calculation in nand_read_subpage Date: Fri, 07 Nov 2008 11:37:57 -0000 From: Werner Almesberger X-Patchwork-Id: 7776 Message-Id: <20081107213757.GC5177@almesberger.net> To: linux-mtd@lists.infradead.org Cc: Alexey Korolev clarify-nand-ecc-access.patch The ECC access calculation in nand_read_subpage is quite hard to understand. This patch makes it more readable. There is a small change in what the algorithm does: while if (eccpos[(start_step + num_steps) * chip->ecc.bytes] & (busw - 1)) looks at the position of the ECC byte following the bytes we're currently reading, aligned_len = ALIGN(eccfrag_len+(pos-aligned_pos), busw); only looks at their length plus the additional data we have to read due to aligning the start position. The latter is more correct than the former, since the next ECC byte could be located anywhere and its location therefore may not give the alignment information we seek. The change also saves 44 bytes on ARM. Signed-off-by: Werner Almesberger Acked-by: Alexey Korolev --- Index: ktrack/drivers/mtd/nand/nand_base.c =================================================================== --- ktrack.orig/drivers/mtd/nand/nand_base.c 2008-11-02 02:28:19.000000000 -0200 +++ ktrack/drivers/mtd/nand/nand_base.c 2008-11-02 02:38:22.000000000 -0200 @@ -851,12 +851,12 @@ } else { /* send the command to read the particular ecc bytes */ /* take care about buswidth alignment in read_buf */ - aligned_pos = eccpos[start_step * chip->ecc.bytes] & ~(busw - 1); - aligned_len = eccfrag_len; - if (eccpos[start_step * chip->ecc.bytes] & (busw - 1)) - aligned_len++; - if (eccpos[(start_step + num_steps) * chip->ecc.bytes] & (busw - 1)) - aligned_len++; + + int pos; + + pos = eccpos[start_step * chip->ecc.bytes]; + aligned_pos = pos & ~(busw - 1); + aligned_len = ALIGN(eccfrag_len+(pos-aligned_pos), busw); chip->cmdfunc(mtd, NAND_CMD_RNDOUT, mtd->writesize + aligned_pos, -1); chip->read_buf(mtd, &chip->oob_poi[aligned_pos], aligned_len);