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

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Date:   Thu, 16 Mar 2023 16:57:54 -0700
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     <linux-kernel@...r.kernel.org>, <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 3/16/23 03:16, Greg Kroah-Hartman wrote:
> On Mon, Mar 13, 2023 at 12:07:27PM -0700, Fenghua Yu wrote:
>> 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?
> 
> Oops, yes, it needs an earlier patch in this series, sorry, I didn't
> call that out properly in the notes section of the patch.
> 
> So I can just take this through my tree if that's ok.

Sure.

Acked-by: Fenghua Yu <fenghua.yu@...el.com>

Thanks.

-Fenghua