mmc: Set pref erase size based on size

If available, eMMC stack uses HC_ERASE_GRP_SIZE as the preferred erase
size. As some high capacity eMMC (64MB) reports this size to 512kB, the
discard operations end up taking very long time.

Improve the behaviour by instead calculating the preferred erase size
based on the eMMC size. In this way the discard operations becomes faster.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
[Ulf: Updated changelog and improved comment in code]
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/core/core.c

@@ -1934,17 +1934,15 @@ void mmc_init_erase(struct mmc_card *card)
 	 * to that size and alignment.
 	 *
 	 * For SD cards that define Allocation Unit size, limit erases to one
-	 * Allocation Unit at a time.  For MMC cards that define High Capacity
-	 * Erase Size, whether it is switched on or not, limit to that size.
-	 * Otherwise just have a stab at a good value.  For modern cards it
-	 * will end up being 4MiB.  Note that if the value is too small, it
-	 * can end up taking longer to erase.
+	 * Allocation Unit at a time.
+	 * For MMC, have a stab at ai good value and for modern cards it will
+	 * end up being 4MiB. Note that if the value is too small, it can end
+	 * up taking longer to erase. Also note, erase_size is already set to
+	 * High Capacity Erase Size if available when this function is called.
 	 */
 	if (mmc_card_sd(card) && card->ssr.au) {
 		card->pref_erase = card->ssr.au;
 		card->erase_shift = ffs(card->ssr.au) - 1;
-	} else if (card->ext_csd.hc_erase_size) {
-		card->pref_erase = card->ext_csd.hc_erase_size;
 	} else if (card->erase_size) {
 		sz = (card->csd.capacity << (card->csd.read_blkbits - 9)) >> 11;
 		if (sz < 128)