Re: [PATCH] mtd: spi-nor: reduce stack usage in spi_nor_parse_sfdp()

[David Laight <david.laight.linux@gmail.com>]

On Mon, 21 Sep 2026 09:59:24 +0100
David Laight <david.laight.linux@gmail.com> wrote:

> On Mon, 21 Sep 2026 09:55:37 +0200
> "Michael Walle" <mwalle@kernel.org> wrote:
> 
> > Hi Arnd,
> > 
> > On Tue Sep 15, 2026 at 9:40 PM CEST, Arnd Bergmann wrote:  
> > > From: Arnd Bergmann <arnd@arndb.de>
> > >
> > > Two large spi_nor_flash_parameter structures on a function stack
> > > is really too much, and this can exceed an otherwise reasonable
> > > frame limit:
> > >
> > > drivers/mtd/spi-nor/sfdp.c: In function 'spi_nor_parse_sfdp':
> > > drivers/mtd/spi-nor/sfdp.c:1717:1: error: the frame size of 1600 bytes is larger than 1536 bytes [-Werror=frame-larger-than=]
> > >
> > > Change one of them to a dynamic allocation to make this more reasonable.    
> > 
> > Thanks for taking care of that. Will you respin the patch with what
> > David suggested?  
> 
> This compiles...

It didn't - I thought I had compiled it :-(
Try:

diff --git a/drivers/mtd/spi-nor/sfdp.c b/drivers/mtd/spi-nor/sfdp.c
index 641f17ad51a0..cec699b52f27 100644
--- a/drivers/mtd/spi-nor/sfdp.c
+++ b/drivers/mtd/spi-nor/sfdp.c
@@ -1519,9 +1519,9 @@ int spi_nor_check_sfdp_signature(struct spi_nor *nor)
  */
 int spi_nor_parse_sfdp(struct spi_nor *nor)
 {
+       struct spi_nor_flash_parameter *save_params __free(kfree) = kmalloc_objs(*save_params, 2);
        const struct sfdp_parameter_header *param_header, *bfpt_header;
        struct sfdp_parameter_header *param_headers = NULL;
-       struct spi_nor_flash_parameter params, params2;
        struct sfdp_header header;
        struct device *dev = nor->dev;
        struct sfdp *sfdp;
@@ -1533,7 +1533,9 @@ int spi_nor_parse_sfdp(struct spi_nor *nor)
         * Get a backup of all the parameter to roll back to in case of an
         * error.
         */
-       memcpy(&params, nor->params, sizeof(params));
+       if (!save_params)
+               return -ENOMEM;
+       save_params[0] = *nor->params;

        /* Get the SFDP header. */
        err = spi_nor_read_sfdp_dma_unsafe(nor, 0, sizeof(header), &header);
@@ -1653,7 +1655,7 @@ int spi_nor_parse_sfdp(struct spi_nor *nor)

        /* Parse optional parameter tables. */
        for (i = 0; i < header.nph; i++) {
-               memcpy(&params2, nor->params, sizeof(params2));
+               save_params[1] = *nor->params;
                param_header = &param_headers[i];

                switch (SFDP_PARAM_HEADER_ID(param_header)) {
@@ -1691,7 +1693,7 @@ int spi_nor_parse_sfdp(struct spi_nor *nor)
                         * spi_nor_flash_parameter data.
                         */
                        err = 0;
-                       memcpy(nor->params, &params2, sizeof(*nor->params));
+                       *nor->params = save_params[1];
                }
        }

@@ -1712,7 +1714,7 @@ int spi_nor_parse_sfdp(struct spi_nor *nor)
 free_param_headers:
        kfree(param_headers);
        if (err)
-               memcpy(nor->params, &params, sizeof(*nor->params));
+               *nor->params = save_params[0];

        return err;
 }
--

David

...
> > 
> > -michael
> >   
> > > Fixes: d20029474a76 ("mtd: spi-nor: push the rollback mechanism into the sfdp module")
> > > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> > > ---
> > >  drivers/mtd/spi-nor/sfdp.c | 10 +++++++---
> > >  1 file changed, 7 insertions(+), 3 deletions(-)    
>