mmc: sh_mmcif: Advance sg_miter before reading blocks

The introduction of sg_miter was a bit sloppy as it didn't
exactly mimic the semantics of the old code on multiblock reads
and writes: these like you to:

- Advance to the first sglist entry *before* starting to read
any blocks from the card.

- Advance and check availability of the next entry *right after*
processing one block.

Not checking if we have more sglist entries right after
reading a block will lead to this not being checked until we
return to the callback to read out more blocks, i.e. until the
next interrupt arrives. Since the last block is the last one
(no more data will arrive) there will not be a next interrupt,
and we will be waiting forever resulting in a timeout for
command 18 when reading multiple blocks.

The same bug was fixed also in the writing of multiple blocks.

Reported-by: Geert Uytterhoeven <[e-mail]>
Fixes: 27b57277d9ba ("mmc: sh_mmcif: Use sg_miter for PIO")
Signed-off-by: Linus Walleij <[e-mail]>
Tested-by: Geert Uytterhoeven <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Ulf Hansson <[e-mail]>

drivers/mmc/host/sh_mmcif.c

···
 static void sh_mmcif_multi_read(struct sh_mmcif_host *host,
 				struct mmc_request *mrq)
 {
+	struct sg_mapping_iter *sgm = &host->sg_miter;
 	struct mmc_data *data = mrq->data;
 
 	if (!data->sg_len || !data->sg->length)
···
 	host->blocksize = sh_mmcif_readl(host->addr, MMCIF_CE_BLOCK_SET) &
 		BLOCK_SIZE_MASK;
 
-	sg_miter_start(&host->sg_miter, data->sg, data->sg_len,
+	sg_miter_start(sgm, data->sg, data->sg_len,
 		       SG_MITER_TO_SG);
+
+	/* Advance to the first sglist entry */
+	if (!sg_miter_next(sgm)) {
+		sg_miter_stop(sgm);
+		return;
+	}
 
 	host->wait_for = MMCIF_WAIT_FOR_MREAD;
 
···
 		return false;
 	}
 
-	if (!sg_miter_next(sgm)) {
-		sg_miter_stop(sgm);
-		return false;
-	}
-
 	p = sgm->addr;
 
 	for (i = 0; i < host->blocksize / 4; i++)
···
 	sgm->consumed = host->blocksize;
 
 	sh_mmcif_bitset(host, MMCIF_CE_INT_MASK, MASK_MBUFREN);
+
+	if (!sg_miter_next(sgm)) {
+		sg_miter_stop(sgm);
+		return false;
+	}
 
 	return true;
 }
···
 static void sh_mmcif_multi_write(struct sh_mmcif_host *host,
 				struct mmc_request *mrq)
 {
+	struct sg_mapping_iter *sgm = &host->sg_miter;
 	struct mmc_data *data = mrq->data;
 
 	if (!data->sg_len || !data->sg->length)
···
 	host->blocksize = sh_mmcif_readl(host->addr, MMCIF_CE_BLOCK_SET) &
 		BLOCK_SIZE_MASK;
 
-	sg_miter_start(&host->sg_miter, data->sg, data->sg_len,
+	sg_miter_start(sgm, data->sg, data->sg_len,
 		       SG_MITER_FROM_SG);
+
+	/* Advance to the first sglist entry */
+	if (!sg_miter_next(sgm)) {
+		sg_miter_stop(sgm);
+		return;
+	}
 
 	host->wait_for = MMCIF_WAIT_FOR_MWRITE;
 
···
 		return false;
 	}
 
-	if (!sg_miter_next(sgm)) {
-		sg_miter_stop(sgm);
-		return false;
-	}
-
 	p = sgm->addr;
 
 	for (i = 0; i < host->blocksize / 4; i++)
 		sh_mmcif_writel(host->addr, MMCIF_CE_DATA, *p++);
 
 	sgm->consumed = host->blocksize;
+
+	if (!sg_miter_next(sgm)) {
+		sg_miter_stop(sgm);
+		return false;
+	}
 
 	sh_mmcif_bitset(host, MMCIF_CE_INT_MASK, MASK_MBUFWEN);