[PATCH] mtd: mtk: avoid warning in mtk_ecc_encode

[PATCH] mtd: mtk: avoid warning in mtk_ecc_encode

[Arnd Bergmann]

When building with -Wmaybe-uninitialized, gcc produces a silly false positive
warning for the mtk_ecc_encode function:

drivers/mtd/nand/mtk_ecc.c: In function 'mtk_ecc_encode':
drivers/mtd/nand/mtk_ecc.c:402:15: error: 'val' may be used uninitialized in this function [-Werror=maybe-uninitialized]

The function for some reason contains a double byte swap on big-endian
builds to get the OOB data into the correct order again, and is written
in a slightly confusing way.

Using a simple memcpy32_fromio() to read the data simplifies it a lot
so it becomes more readable and produces no warning. However, the
output might not have 32-bit alignment, so we have to use another
memcpy to avoid taking alignment faults or writing beyond the end
of the array.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 drivers/mtd/nand/mtk_ecc.c | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/drivers/mtd/nand/mtk_ecc.c b/drivers/mtd/nand/mtk_ecc.c
index d54f666417e1..237c83124a7d 100644
--- a/drivers/mtd/nand/mtk_ecc.c
+++ b/drivers/mtd/nand/mtk_ecc.c
@@ -366,9 +366,9 @@ int mtk_ecc_encode(struct mtk_ecc *ecc, struct mtk_ecc_config *config,
 		   u8 *data, u32 bytes)
 {
 	dma_addr_t addr;
-	u8 *p;
-	u32 len, i, val;
-	int ret = 0;
+	u32 len;
+	u8 eccdata[112];
+	int ret;
 
 	addr = dma_map_single(ecc->dev, data, bytes, DMA_TO_DEVICE);
 	ret = dma_mapping_error(ecc->dev, addr);
@@ -393,14 +393,12 @@ int mtk_ecc_encode(struct mtk_ecc *ecc, struct mtk_ecc_config *config,
 
 	/* Program ECC bytes to OOB: per sector oob = FDM + ECC + SPARE */
 	len = (config->strength * ECC_PARITY_BITS + 7) >> 3;
-	p = data + bytes;
 
-	/* write the parity bytes generated by the ECC back to the OOB region */
-	for (i = 0; i < len; i++) {
-		if ((i % 4) == 0)
-			val = readl(ecc->regs + ECC_ENCPAR(i / 4));
-		p[i] = (val >> ((i % 4) * 8)) & 0xff;
-	}
+	/* write the parity bytes generated by the ECC back to temp buffer */
+	__ioread32_copy(eccdata, ecc->regs + ECC_ENCPAR(0), round_up(len, 4));
+
+	/* copy into possibly unaligned OOB region with actual length */
+	memcpy(data + bytes, eccdata, len);
 timeout:
 
 	dma_unmap_single(ecc->dev, addr, bytes, DMA_TO_DEVICE);
-- 
2.9.0

[PATCH] mtd: mtk: avoid warning in mtk_ecc_encode

[Boris Brezillon]

Hi Arnd,

On Fri, 30 Sep 2016 18:33:02 +0200
Arnd Bergmann <arnd@arndb.de> wrote:

> When building with -Wmaybe-uninitialized, gcc produces a silly false positive
> warning for the mtk_ecc_encode function:
> 
> drivers/mtd/nand/mtk_ecc.c: In function 'mtk_ecc_encode':
> drivers/mtd/nand/mtk_ecc.c:402:15: error: 'val' may be used uninitialized in this function [-Werror=maybe-uninitialized]
> 
> The function for some reason contains a double byte swap on big-endian
> builds to get the OOB data into the correct order again, and is written
> in a slightly confusing way.
> 
> Using a simple memcpy32_fromio() to read the data simplifies it a lot
> so it becomes more readable and produces no warning. However, the
> output might not have 32-bit alignment, so we have to use another
> memcpy to avoid taking alignment faults or writing beyond the end
> of the array.
> 
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  drivers/mtd/nand/mtk_ecc.c | 18 ++++++++----------
>  1 file changed, 8 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/mtd/nand/mtk_ecc.c b/drivers/mtd/nand/mtk_ecc.c
> index d54f666417e1..237c83124a7d 100644
> --- a/drivers/mtd/nand/mtk_ecc.c
> +++ b/drivers/mtd/nand/mtk_ecc.c
> @@ -366,9 +366,9 @@ int mtk_ecc_encode(struct mtk_ecc *ecc, struct mtk_ecc_config *config,
>  		   u8 *data, u32 bytes)
>  {
>  	dma_addr_t addr;
> -	u8 *p;
> -	u32 len, i, val;
> -	int ret = 0;
> +	u32 len;
> +	u8 eccdata[112];
> +	int ret;
>  
>  	addr = dma_map_single(ecc->dev, data, bytes, DMA_TO_DEVICE);
>  	ret = dma_mapping_error(ecc->dev, addr);
> @@ -393,14 +393,12 @@ int mtk_ecc_encode(struct mtk_ecc *ecc, struct mtk_ecc_config *config,
>  
>  	/* Program ECC bytes to OOB: per sector oob = FDM + ECC + SPARE */
>  	len = (config->strength * ECC_PARITY_BITS + 7) >> 3;
> -	p = data + bytes;
>  
> -	/* write the parity bytes generated by the ECC back to the OOB region */
> -	for (i = 0; i < len; i++) {
> -		if ((i % 4) == 0)
> -			val = readl(ecc->regs + ECC_ENCPAR(i / 4));
> -		p[i] = (val >> ((i % 4) * 8)) & 0xff;
> -	}
> +	/* write the parity bytes generated by the ECC back to temp buffer */
> +	__ioread32_copy(eccdata, ecc->regs + ECC_ENCPAR(0), round_up(len, 4));
> +
> +	/* copy into possibly unaligned OOB region with actual length */
> +	memcpy(data + bytes, eccdata, len);

Is it better than

	for (i = 0; i < len; i += 4) {
		u32 val = __raw_readl(ecc->regs + ECC_ENCPAR(i / 4));

		memcpy(data + bytes + i, &val, min(len, 4));
	}

I'm probably missing something, but what's the point of creating a
temporary buffer of 112 bytes on the stack since you'll have to copy
this data to the oob buffer at some point?

>  timeout:
>  
>  	dma_unmap_single(ecc->dev, addr, bytes, DMA_TO_DEVICE);

[PATCH] mtd: mtk: avoid warning in mtk_ecc_encode

[Arnd Bergmann]

On Friday 30 September 2016, Boris Brezillon wrote:
> > +     /* copy into possibly unaligned OOB region with actual length */
> > +     memcpy(data + bytes, eccdata, len);
> 
> Is it better than
> 
>         for (i = 0; i < len; i += 4) {
>                 u32 val = __raw_readl(ecc->regs + ECC_ENCPAR(i / 4));
> 
>                 memcpy(data + bytes + i, &val, min(len, 4));
>         }
> 
> I'm probably missing something, but what's the point of creating a
> temporary buffer of 112 bytes on the stack since you'll have to copy
> this data to the oob buffer at some point?


I tried something like that first, but wasn't too happy with it for
a number of small reasons:

- __raw_readl in a driver is not usually the right API, __memcpy32_from_io
  uses it internally, but it's better for a driver not to rely on that,
  in case we need some barriers (which we may in factt need for other drivers).

- the min(len,4) expression is incorrect, fixing that makes it more complicated
  again

- I didn't like to call memcpy() multiple times, as that might get turned
  into an external function call (the compiler is free to optimize small
  memcpy calls or not).

I agree that he 112 byte buffer isn't ideal either, it just seemed to
be the lesser annoyance.

	Arnd

[PATCH] mtd: mtk: avoid warning in mtk_ecc_encode

[Boris Brezillon]

On Fri, 30 Sep 2016 19:25:17 +0200
Arnd Bergmann <arnd@arndb.de> wrote:

> On Friday 30 September 2016, Boris Brezillon wrote:
> > > +     /* copy into possibly unaligned OOB region with actual length */
> > > +     memcpy(data + bytes, eccdata, len);  
> > 
> > Is it better than
> > 
> >         for (i = 0; i < len; i += 4) {
> >                 u32 val = __raw_readl(ecc->regs + ECC_ENCPAR(i / 4));
> > 
> >                 memcpy(data + bytes + i, &val, min(len, 4));
> >         }
> > 
> > I'm probably missing something, but what's the point of creating a
> > temporary buffer of 112 bytes on the stack since you'll have to copy
> > this data to the oob buffer at some point?  
> 
> 
> I tried something like that first, but wasn't too happy with it for
> a number of small reasons:
> 
> - __raw_readl in a driver is not usually the right API, __memcpy32_from_io
>   uses it internally, but it's better for a driver not to rely on that,
>   in case we need some barriers (which we may in factt need for other drivers).

I agree, even though calling something prefixed with __ (in this case,
__ioread32_copy()) sounds like a bad thing too :).

> 
> - the min(len,4) expression is incorrect, fixing that makes it more complicated
>   again

Sorry, it's min(len - i, 4), which is not that complicated :P.

> 
> - I didn't like to call memcpy() multiple times, as that might get turned
>   into an external function call (the compiler is free to optimize small
>   memcpy calls or not).

Okay.

> 
> I agree that he 112 byte buffer isn't ideal either, it just seemed to
> be the lesser annoyance.

How about we keep your approach, but put the buffer in the mtk_ecc
struct?