linux-kernel - Re: [PATCH 32/36] dmaengine: idxd: use const struct bus

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Date:   Mon, 13 Mar 2023 12:07:27 -0700
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        <linux-kernel@...r.kernel.org>
CC:     <rafael@...nel.org>, Dave Jiang <dave.jiang@...el.com>,
        Vinod Koul <vkoul@...nel.org>, <dmaengine@...r.kernel.org>
Subject: Re: [PATCH 32/36] dmaengine: idxd: use const struct bus_type *

Hi, Greg,

On 3/13/23 11:29, Greg Kroah-Hartman wrote:
> In the functions unbind_store() and bind_store(), a struct bus_type *
> should be a const one, as the driver core bus functions used by this
> variable are expecting the pointer to be constant, and these functions
> do not modify the pointer at all.
> 
> Cc: Fenghua Yu <fenghua.yu@...el.com>
> Cc: Dave Jiang <dave.jiang@...el.com>
> Cc: Vinod Koul <vkoul@...nel.org>
> Cc: dmaengine@...r.kernel.org
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> ---
> Note, this is a patch that is a prepatory cleanup as part of a larger
> series of patches that is working on resolving some old driver core
> design mistakes.  It will build and apply cleanly on top of 6.3-rc2 on
> its own, but I'd prefer if I could take it through my driver-core tree
> so that the driver core changes can be taken through there for 6.4-rc1.
> 
>   drivers/dma/idxd/compat.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/dma/idxd/compat.c b/drivers/dma/idxd/compat.c
> index 3df21615f888..5fd38d1b9d28 100644
> --- a/drivers/dma/idxd/compat.c
> +++ b/drivers/dma/idxd/compat.c
> @@ -16,7 +16,7 @@ extern void device_driver_detach(struct device *dev);
>   
>   static ssize_t unbind_store(struct device_driver *drv, const char *buf, size_t count)
>   {
> -	struct bus_type *bus = drv->bus;
> +	const struct bus_type *bus = drv->bus;
>   	struct device *dev;
>   	int rc = -ENODEV;
>   
> @@ -32,7 +32,7 @@ static DRIVER_ATTR_IGNORE_LOCKDEP(unbind, 0200, NULL, unbind_store);
>   
>   static ssize_t bind_store(struct device_driver *drv, const char *buf, size_t count)
>   {
> -	struct bus_type *bus = drv->bus;
> +	const struct bus_type *bus = drv->bus;
>   	struct device *dev;
>   	struct device_driver *alt_drv = NULL;
>   	int rc = -ENODEV;

After applying this patch, warning is reported:

drivers/dma/idxd/compat.c: In function ‘bind_store’:
drivers/dma/idxd/compat.c:47:47: warning: passing argument 2 of 
‘driver_find’ discards ‘const’ qualifier from pointer target type 
[-Wdiscarded-qualifiers]
    47 |                 alt_drv = driver_find("idxd", bus);
       |                                               ^~~
In file included from ./include/linux/device.h:32,
                  from drivers/dma/idxd/compat.c:6:
./include/linux/device/driver.h:129:59: note: expected ‘struct bus_type 
*’ but argument is of type ‘const struct bus_type *’
   129 |                                          struct bus_type *bus);
       |                                          ~~~~~~~~~~~~~~~~~^~~

Should the "bus" parameter in driver_find() definition be changed to 
const as well to avoid the warning?

Thanks.

-Fenghua