Lukas Wunner <lukas@xxxxxxxxx> writes: > On Tue, Nov 13, 2018 at 09:12:01PM -0800, Florian Fainelli wrote: >> On 11/7/2018 11:06 PM, Lukas Wunner wrote: >> > Here's a first batch of improvements for the spi0 master on the >> > Raspberry Pi. The meat of the series is in its last two patches: >> > >> > * Patch [6/7] allows DMA for transfer buffers starting at an offset not a >> > multiple of 4. This overcomes a limitation affecting Ethernet drivers >> > such as ks8851 which call netdev_alloc_skb_ip_align() to allocate >> > deliberately unaligned receive buffers. >> > >> > * Patch [7/7] speeds up PIO transfers by not polling the RX FIFO when it >> > is known to contain data, or the TX FIFO when it is known to have free >> > space. >> >> Do you have some performance numbers that you could share before/after, >> e.g: transfer latencies, number of interrupts and pure throughput? > > The throughput is primarily determined by the serial clock configured in > the DT for a specific SPI slave. There's nothing we can improve there > in software. > > This series is about reducing CPU usage. E.g. without patch [6/7], > transfer buffers not aligned to 32-bit are transmitted with programmed I/O > instead of DMA. Thus, constantly receiving packets on a ks8851 Ethernet > chip with a serial clock of 20 MHz occupies 25% of a CPU on a RasPi 3 > (for the ks8851 IRQ thread). With the patch, it drops to a negligible > percentage. The spi-bcm2835.c driver currently forces PIO even for > kmalloc'ed buffers which are always contiguous in physical memory, > i.e. for no reason at all. With the whole series, I got an improvement in kmscube on my hx8357d spi panel and vc4 rendering from 5.9fps to 6.6fps. No stats, but the numbers seem fairly stable between runs.
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