Messages in this thread | | | Date | Wed, 15 Apr 2015 20:53:07 +0200 | From | Noralf Trønnes <> | Subject | Re: [PATCH] dmaengine: bcm2835: Add slave dma support |
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Den 15.04.2015 16:37, skrev Martin Sperl: >> On 15.04.2015, at 11:56, Noralf Trønnes <noralf@tronnes.org> wrote: >> +#define MAX_LITE_TRANSFER (SZ_64K - 1) >> +#define MAX_NORMAL_TRANSFER SZ_1G > ... >> + if (c->ch >= 8) /* LITE channel */ >> + max_size = MAX_LITE_TRANSFER; >> + else >> + max_size = MAX_NORMAL_TRANSFER; >> + period_len = min(period_len, max_size); >> + d->frames = (buf_len - 1) / (period_len + 1); > I wonder if it is wise to split the transfers on 65535 bytes for the > Lite DMA-channels - especially if you are transferring to word size > registers (like SPI_FIFO), you still push 16384 words into the register > and the last word of this transfer (word 16384) still is assumed 4 valid > bytes by the device and thus gets operated upon - even if the last byte > contains garbage from the DMA-transfer point of view. > > So maybe it is better to separate on SZ_64K-4 or better still SZ_32K to > be on a power of 2 address boundary.
The datasheet is contradictory: BCM2835 ARM Peripherals - 4.5 DMA LITE Engines 3. The DMA length register is now 16 bits, limiting the maximum transferrable length to 65536 bytes.
A 16-bit register can't hold a value of 65536. Either the max value is 65535 or the register is 17-bits wide.
There is currently no driver that we can use to test >32k buffers. Unless someone disagrees, I will change this back to the SZ_32K value from the original driver (and add a comment to explain that 32k is chosen to stay on the safe side).
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