linux-kernel - Re: [PATCH v2 5/6] esp_scsi: De-duplicate PIO routines

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Date:   Wed, 17 Oct 2018 01:08:54 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Finn Thain <fthain@...egraphics.com.au>
Cc:     Hannes Reinecke <hare@...e.de>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Michael Schmitz <schmitzmic@...il.com>,
        linux-scsi@...r.kernel.org, linux-m68k@...ts.linux-m68k.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/6] esp_scsi: De-duplicate PIO routines

On Tue, Oct 16, 2018 at 10:52:26AM +1100, Finn Thain wrote:
> True enough. We agree that this #ifdef is undesirable. And yet when I 
> tried it, I found an unexpected readability benefit to your suggestion:
> 
> #ifdef CONFIG_SCSI_ESP_PIO
>         u8 __iomem              *fifo_reg;
>         int                     send_cmd_error;
>         u32                     send_cmd_residual;
> #endif
> 
> This grouping does help convey the purpose of these struct members, even 
> though the #ifdef is meant for the compiler not for the human reader.
> 
> So maybe it makes sense to group these definitions (they are all the same 
> size):
> 
>         /* These are used by esp_scsi_send_pio_cmd() */
>         u8 __iomem              *fifo_reg;
>         int                     send_cmd_error;
>         u32                     send_cmd_residual;

I like the grouping, and in fact the ifdef sounds fine to me as well.

> Do static checkers really complain about this? I think the validity of an 
> extern can't be known until the final linkage is done.

None that I know of does, and as said before this patterns is very
common all over the kernel.