Re: [usb-storage] [PATCH 4/7] usb-storage: reject probe of device one non-DMA HCDs when using highmem

[Alan Stern <stern@rowland.harvard.edu>]

On Fri, May 02, 2025 at 07:49:21AM +0100, Christoph Hellwig wrote:
> usb-storage is the last user of the block layer bounce buffering now,
> and only uses it for HCDs that do not support DMA on highmem configs.
> 
> Remove this support and fail the probe so that the block layer bounce
> buffering can go away.

I'm not certain this reasoning is correct.  The code being changed is 
pretty old; it may be that the relevant HCDs now implement bounce 
buffering on their own.

However, the combination of USB mass storage with these restricted host 
controllers is probably pretty rare.

> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  drivers/usb/storage/usb.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/usb/storage/usb.c b/drivers/usb/storage/usb.c
> index d36f3b6992bb..49bbfe4610d5 100644
> --- a/drivers/usb/storage/usb.c
> +++ b/drivers/usb/storage/usb.c
> @@ -1057,12 +1057,15 @@ int usb_stor_probe1(struct us_data **pus,
>  
>  	/*
>  	 * Some USB host controllers can't do DMA; they have to use PIO.

I'd like to see this part of the comment updated:

	 * Some USB host controllers can't do DMA: They have to use PIO,
	 * or they have to use a small dedicated local memory area, or 
	 * they have other restrictions on addressable memory.

That explains the reason for the check of hcd->localmem_pool.

> -	 * For such controllers we need to make sure the block layer sets
> -	 * up bounce buffers in addressable memory.
> +	 * We can't support these controllers on highmem systems as the
> +	 * usb-storage code lacks the code to kmap or bounce buffer.

This looks a little stange.  How about instead:

	... as we don't kmap or bounce buffers.

>  	 */
> -	if (!hcd_uses_dma(bus_to_hcd(us->pusb_dev->bus)) ||
> -	    bus_to_hcd(us->pusb_dev->bus)->localmem_pool)
> -		host->no_highmem = true;
> +	if (IS_ENABLED(CONFIG_HIGHMEM) &&
> +	    (!hcd_uses_dma(bus_to_hcd(us->pusb_dev->bus)) ||
> +	     bus_to_hcd(us->pusb_dev->bus)->localmem_pool)) {
> +		dev_warn(&intf->dev, "USB Mass Storage device not support on this HCD\n");

Please say "host controller" instead of "HCD", and delete "device"
(and say "supported" instead of "support").

More importantly, set result to a negative error value before returning 
so that it won't look like the probe succeeded.

> +		goto release;
> +	}
>  
>  	/* Get the unusual_devs entries and the descriptors */
>  	result = get_device_info(us, id, unusual_dev);
> @@ -1081,6 +1084,7 @@ int usb_stor_probe1(struct us_data **pus,
>  
>  BadDevice:
>  	usb_stor_dbg(us, "storage_probe() failed\n");
> +release:
>  	release_everything(us);
>  	return result;
>  }

Alan Stern