linux-kernel - Re: [PATCH] usb: storage: ene_ub6250: Fix right shift warnings

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Message-ID: <b35a344a-018b-44ae-975a-7767a3d5b6ec@rowland.harvard.edu>
Date: Wed, 31 Jul 2024 14:19:54 -0400
From: Alan Stern <stern@...land.harvard.edu>
To: Abhishek Tamboli <abhishektamboli9@...il.com>
Cc: gregkh@...uxfoundation.org, oneukum@...e.com,
	usb-storage@...ts.one-eyed-alien.net, linux-usb@...r.kernel.org,
	skhan@...uxfoundation.org, dan.carpenter@...aro.org,
	rbmarliere@...il.com,
	linux-kernel-mentees@...ts.linuxfoundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb: storage: ene_ub6250: Fix right shift warnings

On Wed, Jul 31, 2024 at 11:34:45PM +0530, Abhishek Tamboli wrote:
> On Wed, Jul 31, 2024 at 10:04:33AM -0400, Alan Stern wrote:
> > On Wed, Jul 31, 2024 at 11:15:28AM +0200, 'Oliver Neukum' via USB Mass Storage on Linux wrote:
> > > Hi,
> > > 
> > > On 30.07.24 19:56, Abhishek Tamboli wrote:
> > > > On Tue, Jul 30, 2024 at 09:09:05AM +0200, Oliver Neukum wrote:
> > > 
> > > > > 1. use a constant, where a constant is used
> > > > I think you are suggesting that I should replace hard-coded values like the
> > > > buffer size with named constants. For example:
> > > > 
> > > > #define BUF_SIZE 8
> > > > unsigned char buf[BUF_SIZE];
> > > 
> > > Yes, but the constant we need to look at here is bl_len.
> > > This is a variable needlessly.
> > 
> > The code in ms_scsi_read_capacity() is written that way to be consistent 
> > with the sd_scsi_read_capacity() routine.  Or maybe it was just 
> > copied-and-pasted, and then the variable's type was changed for no good 
> > reason.
> > 
> > Replacing the variable with a constant won't make much difference.  The 
> > compiler will realize that bl_len has a constant value and will generate 
> > appropriate code anyway.  I think just changing the type is a fine fix.
> > 
> > > > > 2. use the macros for converting endianness
> > > > Can I use macros like cpu_to_le32 for converting the bl_num and bl_len values.
> > > > Should I replace all instances of manual bitwise shifts with these macros?
> > > 
> > > Yes.
> > > 
> > > > For example:
> > > > 
> > > >      u32 bl_len = 0x200;
> > > >      buf[0] = cpu_to_le32(bl_num) >> 24;
> > > >      buf[4] = cpu_to_le32(bl_len) >> 24;
> > > > 
> > > > Is using cpu_to_le32 appropriate for the data format required by this
> > > > device?
> > > 
> > > Well, the format is big endian. So, cpu_to_be32() will be required.
> > 
> > Better yet, use put_unaligned_be32().
> Would you recommend submitting a follow-up patch to incorporate this change, or should I leave it as is for now.

You can submit another patch as a clean-up, if you want.  But as I said, 
it isn't needed.

> >However, there's nothing really 
> >wrong with the code as it stands. It doesn't need to be changed now.
> As you mentioned there's no need to change the code, So my initial patch is okay as is?

It is as far as I'm concerned.  Obviously Oliver has a different 
opinion.  But I'm the Maintainer of the usb-storage driver, so my 
opinion counts for more than his does, in this case.  :-)

Alan Stern