On Fri, Dec 20, 2024 at 09:46:27PM +0800, Zhongqiu Han wrote: > On 12/19/2024 9:28 PM, Greg KH wrote: > > On Thu, Dec 19, 2024 at 08:42:54PM +0800, Zhongqiu Han wrote: > > > It is considered good practice to call cpu_relax() in busy loops, see > > > Documentation/process/volatile-considered-harmful.rst. This can lower CPU > > > power consumption or yield to a hyperthreaded twin processor, or serve as > > > a compiler barrier. In addition, if something goes wrong in the busy loop > > > at least it can prevent things from getting worse. > > > > > > Signed-off-by: Zhongqiu Han <quic_zhonhan@xxxxxxxxxxx> > > > --- > > > drivers/tty/mips_ejtag_fdc.c | 4 ++-- > > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > > > diff --git a/drivers/tty/mips_ejtag_fdc.c b/drivers/tty/mips_ejtag_fdc.c > > > index afbf7738c7c4..b17ead1e9698 100644 > > > --- a/drivers/tty/mips_ejtag_fdc.c > > > +++ b/drivers/tty/mips_ejtag_fdc.c > > > @@ -346,7 +346,7 @@ static void mips_ejtag_fdc_console_write(struct console *c, const char *s, > > > /* Busy wait until there's space in fifo */ > > > while (__raw_readl(regs + REG_FDSTAT) & REG_FDSTAT_TXF) > > > - ; > > > + cpu_relax(); > > > __raw_writel(word.word, regs + REG_FDTX(c->index)); > > > } > > > out: > > > @@ -1233,7 +1233,7 @@ static void kgdbfdc_push_one(void) > > > /* Busy wait until there's space in fifo */ > > > while (__raw_readl(regs + REG_FDSTAT) & REG_FDSTAT_TXF) > > > - ; > > > + cpu_relax(); > > > > How did you test this? Are you _sure_ it is needed at all? I think you > > just made these loops take a lot longer than before :( > > > > Have you had problems with these tight loops doing anything bad to your > > system? > > > > thanks, > > > > greg k-h > > Hi Greg, > Thanks a lot for the review~ > > Perhaps I should submit an RFC patch and explain the situation, as I > don't have a MIPS device for testing. Indeed, the cpu_relax() > implementation for MIPS is a memory barrier, which, compared to busy > waiting, doesn't save power and can make loops slower than before. > However, according to its definition file, for certain MIPS-based > architectures like Loongarch-3, it can help force the Loongson-3 SFB > (Store-Fill-Buffer) flush to avoid pending writes. Below is the > implementation of cpu_relax() for the MIPS architecture and its > comments. > > ----------------------------------------------------------------- > arch/mips/include/asm/vdso/processor.h > > #ifdef CONFIG_CPU_LOONGSON64 > /* > * Loongson-3's SFB (Store-Fill-Buffer) may buffer writes indefinitely > * when a tight read loop is executed, because reads take priority over > * writes & the hardware (incorrectly) doesn't ensure that writes will > * eventually occur. > * > * Since spin loops of any kind should have a cpu_relax() in them, force > * an SFB flush from cpu_relax() such that any pending writes will > * become visible as expected. > */ > #define cpu_relax() smp_mb() > #else > #define cpu_relax() barrier() > #endif > ---------------------------------------------------------------- > > Based on this, cpu_relax() should be needed here? :) I don't know, please test and let us know! Without testing of this on real hardware, we can't take this change for obvious reasons. thanks, greg k-h
