Re: [PATCH v3 0/4] tty: TX helpers

[Ilpo Järvinen]

On Wed, 7 Sep 2022, Greg Kroah-Hartman wrote:

> On Wed, Sep 07, 2022 at 03:21:28PM +0300, Ilpo Järvinen wrote:
> > On Wed, 7 Sep 2022, Arnd Bergmann wrote:
> > 
> > > On Wed, Sep 7, 2022, at 12:16 PM, Ilpo Järvinen wrote:
> > > > On Wed, 7 Sep 2022, Jiri Slaby wrote:
> > > >> On 06. 09. 22, 13:30, Johan Hovold wrote:
> > > >> > On Tue, Sep 06, 2022 at 12:48:01PM +0200, Jiri Slaby wrote:
> > > >> > NAK
> > > >> 
> > > >> I'd love to come up with something nicer. That would be a function in
> > > >> serial-core calling hooks like I had [1] for example. But provided all those
> > > >> CPU workarounds/thunks, it'd be quite expensive to call two functions per
> > > >> character.
> > > >> 
> > > >> Or creating a static inline (having ± the macro content) and the hooks as
> > > >> parameters and hope for optimizations to eliminate thunks (also suggested in
> > > >> the past [1]).
> > > >> 
> > > >> [1] https://lore.kernel.org/all/20220411105405.9519-1-jslaby@xxxxxxx/
> > > >
> > > > I second Jiri here.
> > > >
> > > > Saving lines in drivers is not that important compared with all removing 
> > > > all the variants of the same thing that have crept there over the years.
> > > >
> > > > I suspect the main reason for the variants is that everybody just used 
> > > > other drivers as examples and therefore we've a few "main" variant 
> > > > branches depending on which of the drivers was used as an example for the 
> > > > other. That is hardly a good enough reason to keep them different and as 
> > > > long as each driver keeps its own function for this, it will eventually 
> > > > lead to similar differentiation so e.g. a one-time band-aid similarization 
> > > > would not help in the long run.
> > > >
> > > > Also, I don't understand why you see it unreadable when the actual code is 
> > > > out in the open in that macro. It's formatted much better than e.g. 
> > > > read_poll_timeout() if you want an example of something that is hardly 
> > > > readable ;-). I agree though there's a learning-curve, albeit small, that 
> > > > it actually creates a function but that doesn't seem to me as big of an 
> > > > obstacle you seem to think.
> > > 
> > > I think it would help to replace the macro that defines
> > > the function with a set of macros that can be used in
> > > function bodies. This would avoid the __VA_ARGS__ stuff
> > > and allow readers that are unfamiliar with tty drivers to
> > > treat it as a function call.
> > > 
> > > So e.g. instead of 
> > > 
> > > static DEFINE_UART_PORT_TX_HELPER_LIMITED(altera_jtaguart_do_tx_chars,
> > > 		true,
> > > 		writel(ch, port->membase + ALTERA_JTAGUART_DATA_REG),
> > > 		({}));
> > > 
> > > the altera_jtaguart driver would contain a function like
> > > 
> > > static int altera_jtaguart_do_tx_chars(struct uart_port *port,
> > >                                        unsigned int count)
> > > {
> > >        char ch;
> > > 
> > >        return uart_port_tx_helper_limited(port, ch, count, true,
> > >                 writel(ch, port->membase + ALTERA_JTAGUART_DATA_REG),
> > >                 ({}));
> > > }
> > > 
> > > or some variation of that. It's a few more lines, but those
> > > extra lines would help me understand what is actually going on
> > > while still avoiding the usual bugs and duplication.
> > > 
> > > If the caller of that function is itself trivial (like
> > > serial21285_tx_chars), then the intermediate function can
> > > be omitted in order to save some of the extra complexity.
> > 
> > I'd be ok with that. There's still a small startle factor associated to 
> > passing that writel(...) as an argument to a "function" but it's the same 
> > for other things such as read_poll_timeout() so not an end of the world.
> 
> That's going to incure the function-pointer-indirection-call for every
> character that Jiri's original submission had, so I don't think this is
> a very viable solution, sorry.

I don't think you got what Arnd meant. It must still be technically a 
#define because you cannot pass
	writel(ch, port->membase + ALTERA_JTAGUART_DATA_REG)
as an argument to a real function like he did in the example above.
It's similar to how read_poll_timeout() and friends are #defines despite 
being lowercased.

-- 
 i.