Re: [PATCH v6 03/24] rtw89: add core and trx files

[Arnd Bergmann <arnd@arndb.de>]

On Tue, Oct 5, 2021 at 9:46 AM Kalle Valo <kvalo@codeaurora.org> wrote:
> Pkshih <pkshih@realtek.com> writes:
> >> From: kvalo=codeaurora.org@mg.codeaurora.org
> >>
> >> > +static __always_inline void RTW89_SET_TXWD(u8 *txdesc, u32 val,
> >> > u8 offset, u32 mask)
> >> > +{
> >> > +  u32 *txd32 = (u32 *)txdesc;
> >> > +
> >> > +  le32p_replace_bits((__le32 *)(txd32 + offset), val, mask);
> >> > +}
> >>
> >> I'm not convinced about this either, please just use inline.
> >
> > This is because 'mask' argument of le32p_replace_bits() must be constant
> > only. If I use inline and build this driver with ccflags-y += -Os,
> > compiler reports errors:
> >
> > In function 'field_multiplier',
> >     inlined from 'le32_encode_bits' at ./include/linux/bitfield.h:154:1,
> >     inlined from 'le32p_replace_bits' at ./include/linux/bitfield.h:154:1,
> >     inlined from 'RTW89_SET_FWCMD_UA32.constprop' at /work/git-root/rtwlan/rtw89/fw.h:1397:2:
> > ./include/linux/bitfield.h:119:3: error: call to '__bad_mask' declared with attribute error: bad bitfield mask
> >   119 |   __bad_mask();
> >       |   ^~~~~~~~~~~~
> >
> > I check the implement of le32p_replace_bits(), it looks like
> >
> > static __always_inline void type##p_replace_bits(__##type *p,           \
> >                                         base val, base field)           \
> > {                                                                       \
> >         *p = (*p & ~to(field)) | type##_encode_bits(val, field);        \
> > }
> >
> > So, I imitate the function to use __always_inline, and then it works.
> >
> > Do you think I don't need to consider the case of Os?
> > But, -Os seems a standard option of Linux kernel.
> >
> > ifdef CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE
> > KBUILD_CFLAGS += -O2
> > else ifdef CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3
> > KBUILD_CFLAGS += -O3
> > else ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
> > KBUILD_CFLAGS += -Os
> > endif
>
> Yeah, we need to support -Os.
>
> Arnd, what do you suggest? Is __always_inline good solution for this? I
> think we should at least add a comment explaining why it's needed.

__always_inline can make sense to force the compiler to behave
sanely if it doesn't work it out by itself, and I think that is how this
function was meant to be used: the __compiletime_error in bitfield.h
is intended to find any callers that have a non-constant argument,
because that would result in horrible code.

I would suggest looking at the object code that you get with -Os after
the added __always_inline, just to make sure that this isn't also
horrible.

Looking at the driver code, as in

+#define RTW89_SET_TXWD_BODY_WP_OFFSET(txdesc, val) \
+ RTW89_SET_TXWD(txdesc, val, 0x00, GENMASK(31, 24))
+#define RTW89_SET_TXWD_BODY_MORE_DATA(txdesc, val) \
+ RTW89_SET_TXWD(txdesc, val, 0x00, BIT(23))
+#define RTW89_SET_TXWD_BODY_WD_INFO_EN(txdesc, val) \
+ RTW89_SET_TXWD(txdesc, val, 0x00, BIT(22))
+#define RTW89_SET_TXWD_BODY_FW_DL(txdesc, val) \
+ RTW89_SET_TXWD(txdesc, val, 0x00, BIT(20))

I would personally write this without the wrappers, instead defining the
bitmask macros as the masks and then open-coding the
le32p_replace_bits() calls instead, which I would find more
intuitive while it avoids the problem with the bitmasks.

Going back one more step, I see that that rtw89_core_fill_txdesc()
manipulates the descriptor fields in-memory, which also seems
like a bad idea: The descriptor is mapped as cache-coherent,
so on machines with no coherent DMA (i.e. most ARM or MIPS
machines), that is uncached memory, and writing the descriptor
using a series of read-modify-write cycles on uncached memory
will be awfully slow. Maybe the answer is to just completely
replace the descriptor access.

       Arnd