Messages in this thread | | | Subject | Re: [PATCH] serial: vt8500_serial: Fix a parameter of find_first_zero_bit. | From | Christophe JAILLET <> | Date | Tue, 23 Aug 2016 22:24:22 +0200 |
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Le 23/08/2016 à 11:23, Arnd Bergmann a écrit : > On Tuesday, August 23, 2016 6:20:28 AM CEST Christophe JAILLET wrote: >> Le 22/08/2016 à 10:42, Arnd Bergmann a écrit : >>> [...] >>> Sorry, but I'm not following the logic here. >>> >>> [...] >>> You argue that the two have the same meaning, which I see, but >>> why is it better than the existing code? >>> >>> Arnd >> Hi, >> >> sorry if my explanation was unclear. >> >> What I mean is that if "sizeof(unsigned long) = 4" (i.e. 32 bits systems >> ?) then: >> >> port = find_first_zero_bit(&vt8500_ports_in_use, sizeof(vt8500_ports_in_use)); >> turns into: >> port = find_first_zero_bit(&vt8500_ports_in_use, 4); >> >> find_first_zero_bit "Returns the bit number of the first set bit. If no bits are set, returns @size." >> So, in this case, it can return 1, 2, 3 or 4, if one of the 4 first bits is 0. >> And will also return 4, if none of the 4 first bits is 0. > Ah, got it. > >> Finally, what I meant by "Other options are possible:" is: >> - 'vt8500_ports_in_use' being a 'unsigned long', use ffz to reduce code verbosity >> port = ffz(&vt8500_ports_in_use); >> would also work, because it is equivalent to: >> port = find_first_zero_bit(&vt8500_ports_in_use, BITS_PER_LONG); >> >> - VT8500_MAX_PORTS, in order to be consistent with the test below >> port = find_first_zero_bit(&vt8500_ports_in_use, VT8500_MAX_PORTS); >> would also work and is maybe more logical in regard to the test "if (port >= VT8500_MAX_PORTS)" >> >> >> >> Now if "sizeof(unsigned long) = 8" (i.e. 64 bits systems ?), the actual code would work. >> But using "sizeof(long)" to mean "more than VT8500_MAX_PORTS" is odd. >> In other words, expressing a number of bits using something that gives a size in bytes is, IMHO, spurious. >> >> All this is pure speculation. >> >> Hoping that it is clearer now ( and that my analysis is right :) ) > I misread the code in the same way the original author wrote it wrong, > I guess it was meant to say > > port = find_first_zero_bit(&vt8500_ports_in_use, sizeof(vt8500_ports_in_use) * 8); I guess so.
> to convert number of bytes into number of bits. Your patch is absolutely > correct, but being more specific about the kind of mistake that was made > is a good idea. > > Regarding which of the four alternatives to use, I'd probably use > your third one, checking against VT8500_MAX_PORTS. To make this code > absolutely foolproof, we can add this hunk too then: Agreed for VT8500_MAX_PORTS. This documents the code.
Using DECLARE_BITMAP is also nice (even if I doubt that it will be useful one day in this particular case) It would turn the vt8500_ports_in_use variable into a pointer. So some more code modification would be required.
Thk for your feedback and comments.
I'll send a v2.
CJ
> diff --git a/drivers/tty/serial/vt8500_serial.c b/drivers/tty/serial/vt8500_serial.c > index 23cfc5e16b45..a68be66d2770 100644 > --- a/drivers/tty/serial/vt8500_serial.c > +++ b/drivers/tty/serial/vt8500_serial.c > @@ -118,7 +118,7 @@ struct vt8500_port { > * have been allocated as we can't use pdev->id in > * devicetree > */ > -static unsigned long vt8500_ports_in_use; > +static DECLARE_BITMAP(vt8500_ports_in_use, VT8500_MAX_PORTS); > > static inline void vt8500_write(struct uart_port *port, unsigned int val, > unsigned int off) > > > > Arnd
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