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SubjectRe: [PATCH v2 5/6] esp_scsi: De-duplicate PIO routines
On Mon, 15 Oct 2018, Hannes Reinecke wrote:

> >
> > In the case of send_cmd_residual, that would mean a second #ifdef
> > added to esp_data_bytes_sent() where it gets used. I'm happy to comply
> > but I fear that all these #ifdefs may harm readability...
> >
> > There are already other variables in struct esp that may go unused,
> > such as dma_regs, that don't have #ifdefs to elide them. Are these
> > also problematic in some way?
> >
> The unused fields in the struct are not so much an issue; in fact, it
> rather complicated things when having individual fields in the struct
> surrounded by CONFIG_XXX, as then the order of the fields would change
> depending on the configuration. Which makes it really hard to debug ..
>

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;

> However, the function declaration really is a worry, as the actual
> function body only exists when the config option is enabled. So either
> add a dummy function or surround the function declaration by
> CONFIG_ESP_PIO.
> Otherwise I think Dan Carpenter and the likes are guaranteed to send you
> a nice mail complaining about this ...
>

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

At that point the checker may complain that no compilation unit references
a symbol in a header.

But this would lead to false positives where a header file is shared by
separate programs which share library code but not macros.

--

> Cheers,
>
> Hannes
>

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