Re: [PATCH v4 4/6] mtd: rawnand: add NVIDIA Tegra NAND Flash controller driver

[Boris Brezillon <boris.brezillon@bootlin.com>]

On Tue, 12 Jun 2018 10:06:42 +0200
Stefan Agner <stefan@agner.ch> wrote:

> On 12.06.2018 02:03, Dmitry Osipenko wrote:
> > On Monday, 11 June 2018 23:52:22 MSK Stefan Agner wrote:  
> >> Add support for the NAND flash controller found on NVIDIA
> >> Tegra 2 SoCs. This implementation does not make use of the
> >> command queue feature. Regular operations/data transfers are
> >> done in PIO mode. Page read/writes with hardware ECC make
> >> use of the DMA for data transfer.
> >>
> >> Signed-off-by: Lucas Stach <dev@lynxeye.de>
> >> Signed-off-by: Stefan Agner <stefan@agner.ch>
> >> ---
> >>  MAINTAINERS                       |    7 +
> >>  drivers/mtd/nand/raw/Kconfig      |    6 +
> >>  drivers/mtd/nand/raw/Makefile     |    1 +
> >>  drivers/mtd/nand/raw/tegra_nand.c | 1248 +++++++++++++++++++++++++++++
> >>  4 files changed, 1262 insertions(+)
> >>  create mode 100644 drivers/mtd/nand/raw/tegra_nand.c
> >>  
> [snip]
> >> +static int tegra_nand_cmd(struct nand_chip *chip,
> >> +			 const struct nand_subop *subop)
> >> +{
> >> +	const struct nand_op_instr *instr;
> >> +	const struct nand_op_instr *instr_data_in = NULL;
> >> +	struct tegra_nand_controller *ctrl = to_tegra_ctrl(chip->controller);
> >> +	unsigned int op_id, size = 0, offset = 0;
> >> +	bool first_cmd = true;
> >> +	u32 reg, cmd = 0;
> >> +	int ret;
> >> +
> >> +	for (op_id = 0; op_id < subop->ninstrs; op_id++) {
> >> +		unsigned int naddrs, i;
> >> +		const u8 *addrs;
> >> +		u32 addr1 = 0, addr2 = 0;
> >> +
> >> +		instr = &subop->instrs[op_id];
> >> +
> >> +		switch (instr->type) {
> >> +		case NAND_OP_CMD_INSTR:
> >> +			if (first_cmd) {
> >> +				cmd |= COMMAND_CLE;
> >> +				writel_relaxed(instr->ctx.cmd.opcode,
> >> +					       ctrl->regs + CMD_REG1);
> >> +			} else {
> >> +				cmd |= COMMAND_SEC_CMD;
> >> +				writel_relaxed(instr->ctx.cmd.opcode,
> >> +					       ctrl->regs + CMD_REG2);
> >> +			}
> >> +			first_cmd = false;
> >> +			break;
> >> +		case NAND_OP_ADDR_INSTR:
> >> +			offset = nand_subop_get_addr_start_off(subop, op_id);
> >> +			naddrs = nand_subop_get_num_addr_cyc(subop, op_id);
> >> +			addrs = &instr->ctx.addr.addrs[offset];
> >> +
> >> +			cmd |= COMMAND_ALE | COMMAND_ALE_SIZE(naddrs);
> >> +			for (i = 0; i < min_t(unsigned int, 4, naddrs); i++)
> >> +				addr1 |= *addrs++ << (BITS_PER_BYTE * i);
> >> +			naddrs -= i;
> >> +			for (i = 0; i < min_t(unsigned int, 4, naddrs); i++)
> >> +				addr2 |= *addrs++ << (BITS_PER_BYTE * i);
> >> +			writel_relaxed(addr1, ctrl->regs + ADDR_REG1);
> >> +			writel_relaxed(addr2, ctrl->regs + ADDR_REG2);
> >> +			break;
> >> +
> >> +		case NAND_OP_DATA_IN_INSTR:
> >> +			size = nand_subop_get_data_len(subop, op_id);
> >> +			offset = nand_subop_get_data_start_off(subop, op_id);
> >> +
> >> +			cmd |= COMMAND_TRANS_SIZE(size) | COMMAND_PIO |
> >> +				COMMAND_RX | COMMAND_A_VALID;
> >> +
> >> +			instr_data_in = instr;
> >> +			break;
> >> +
> >> +		case NAND_OP_DATA_OUT_INSTR:
> >> +			size = nand_subop_get_data_len(subop, op_id);
> >> +			offset = nand_subop_get_data_start_off(subop, op_id);
> >> +
> >> +			cmd |= COMMAND_TRANS_SIZE(size) | COMMAND_PIO |
> >> +				COMMAND_TX | COMMAND_A_VALID;
> >> +
> >> +			memcpy(&reg, instr->ctx.data.buf.out + offset, size);
> >> +			writel_relaxed(reg, ctrl->regs + RESP);
> >> +
> >> +			break;
> >> +		case NAND_OP_WAITRDY_INSTR:
> >> +			cmd |= COMMAND_RBSY_CHK;
> >> +			break;
> >> +
> >> +		}
> >> +	}
> >> +
> >> +	cmd |= COMMAND_GO | COMMAND_CE(ctrl->cur_cs);
> >> +	writel_relaxed(cmd, ctrl->regs + COMMAND);
> >> +	ret = wait_for_completion_io_timeout(&ctrl->command_complete,
> >> +					     msecs_to_jiffies(500));  
> > 
> > It's not obvious to me whether _io_ variant is appropriate to use here, would 
> > be nice if somebody could clarify that. Maybe block/ already does the IO 
> > accounting itself and hence the IO time would be counted twice in that case.  
> 
> Good that you bring this up.
> 
> I don't think that there is any higher layer which could take care of
> accounting. Usually, with raw nand there is no block layer involved
> anyway.
> 
> In a quick test it seems that only when using wait_for_completion_io I/O
> is properly accounted in the "wait" section of top.
> 
> So far only a single driver (omap2) used the _io variant, but I think it
> is the right thing to do! After all, it is I/O...
> 
> Boris or any other MTD maintainer, any comment on this?

Given this definition of io_schedule_timeout() [1] (which is used when
you call wait_for_completion_io_timeout()), I'd say it's not useful to
use the _io_ version, simply because MTD devs are not exposed as blk
devices, and thus don't need the blk_schedule_flush_plug() that is done
is io_schedule_prepare(). But that also means MTD I/Os are not
accounted as I/Os :-(.

Let's go for the non-io version for now, since all drivers except omap2
seem to use this function.

[1]https://elixir.bootlin.com/linux/v4.17.1/source/kernel/sched/core.c#L5164