Re: [PATCH v6 2/2] mtd: rawnand: meson: add support for Amlogic NAND flash controller

[Boris Brezillon]

On Mon, 12 Nov 2018 17:54:16 +0100
Boris Brezillon <boris.brezillon@xxxxxxxxxxx> wrote:

> +Wolfram to give some inputs on the DMA issue.
> 
> On Mon, 12 Nov 2018 17:13:51 +0100
> Miquel Raynal <miquel.raynal@xxxxxxxxxxx> wrote:
> 
> > Hello,
> > 
> > Boris Brezillon <boris.brezillon@xxxxxxxxxxx> wrote on Tue, 6 Nov 2018
> > 11:22:06 +0100:
> >   
> > > On Tue, 6 Nov 2018 18:00:37 +0800
> > > Liang Yang <liang.yang@xxxxxxxxxxx> wrote:
> > >     
> > > > On 2018/11/6 17:28, Boris Brezillon wrote:      
> > > > > On Tue, 6 Nov 2018 17:08:00 +0800
> > > > > Liang Yang <liang.yang@xxxxxxxxxxx> wrote:
> > > > >         
> > > > >> On 2018/11/5 23:53, Boris Brezillon wrote:        
> > > > >>> On Fri, 2 Nov 2018 00:42:21 +0800
> > > > >>> Jianxin Pan <jianxin.pan@xxxxxxxxxxx> wrote:
> > > > >>>            
> > > > >>>> +
> > > > >>>> +static inline u8 meson_nfc_read_byte(struct mtd_info *mtd)
> > > > >>>> +{
> > > > >>>> +	struct nand_chip *nand = mtd_to_nand(mtd);
> > > > >>>> +	struct meson_nfc *nfc = nand_get_controller_data(nand);
> > > > >>>> +	u32 cmd;
> > > > >>>> +
> > > > >>>> +	cmd = nfc->param.chip_select | NFC_CMD_DRD | 0;
> > > > >>>> +	writel(cmd, nfc->reg_base + NFC_REG_CMD);
> > > > >>>> +
> > > > >>>> +	meson_nfc_drain_cmd(nfc);        
> > > > >>>
> > > > >>> You probably don't want to drain the FIFO every time you read a byte on
> > > > >>> the bus, and I guess the INPUT FIFO is at least as big as the CMD
> > > > >>> FIFO, right? If that's the case, you should queue as much DRD cmd as
> > > > >>> possible and only sync when the user explicitly requests it or when
> > > > >>> the INPUT/READ FIFO is full.
> > > > >>>            
> > > > >> Register 'NFC_REG_BUF' can holds only 4 bytes, also DRD sends only one
> > > > >> nand cycle to read one byte and covers the 1st byte every time reading.
> > > > >> i think nfc controller is faster than nand cycle, but really it is not
> > > > >> high efficiency when reading so many bytes once.
> > > > >> Or use dma command here like read_page and read_page_raw.        
> > > > > 
> > > > > Yep, that's also an alternative, though you'll have to make sure the
> > > > > buffer passed through the nand_op_inst is DMA-safe, and use a bounce
> > > > > buffer when that's not the case.
> > > > >         
> > > > ok, i will try dma here.      
> > > 
> > > We should probably expose the bounce buf handling as generic helpers at
> > > the rawnand level:
> > > 
> > > void *nand_op_get_dma_safe_input_buf(struct nand_op_instr *instr)
> > > {
> > > 	void *buf;
> > > 
> > > 	if (WARN_ON(instr->type != NAND_OP_DATA_IN_INSTR))
> > > 		return NULL;
> > > 
> > > 	if (virt_addr_valid(instr->data.in) &&
> > > 	    !object_is_on_stack(instr->data.buf.in))
> > > 		return instr->data.buf.in;
> > > 
> > > 	return kzalloc(instr->data.len, GFP_KERNEL);
> > > }
> > > 
> > > void nand_op_put_dma_safe_input_buf(struct nand_op_instr *instr,
> > > 				    void *buf)
> > > {
> > > 	if (WARN_ON(instr->type != NAND_OP_DATA_IN_INSTR) ||
> > > 	    WARN_ON(!buf))
> > > 		return;
> > > 
> > > 	if (buf == instr->data.buf.in)
> > > 		return;
> > > 
> > > 	memcpy(instr->data.buf.in, buf, instr->data.len);
> > > 	kfree(buf);
> > > }
> > > 
> > > const void *nand_op_get_dma_safe_output_buf(struct nand_op_instr *instr)
> > > {
> > > 	void *buf;
> > > 
> > > 	if (WARN_ON(instr->type != NAND_OP_DATA_OUT_INSTR))
> > > 		return NULL;
> > > 
> > > 	if (virt_addr_valid(instr->data.out) &&
> > > 	    !object_is_on_stack(instr->data.buf.out))
> > > 		return instr->data.buf.out;
> > > 
> > > 	return kmemdup(instr->data.buf.out, GFP_KERNEL);
> > > }
> > > 
> > > void nand_op_put_dma_safe_output_buf(struct nand_op_instr *instr,
> > > 				    void *buf)
> > > {
> > > 	if (WARN_ON(instr->type != NAND_OP_DATA_OUT_INSTR) ||
> > > 	    WARN_ON(!buf))
> > > 		return;
> > > 
> > > 	if (buf != instr->data.buf.out)
> > > 		kfree(buf);
> > > }    
> > 
> > Not that I am against such function, but maybe they should come with
> > comments stating that there is no reliable way to find if a buffer is
> > DMA-able at runtime and these are just sanity checks (ie. required, but
> > probably not enough).  
> 
> It's not 100% reliable, but it should cover most cases. Note that the
> NAND framework already uses virt_addr_valid() to decide when to use its
> internal bounce buffer, so this should be fixed too if we want a fully
> reliable solution.
> 
> > This is my understanding of Wolfram's recent talk
> > at ELCE [1].  
> 
> Yes, you're right, but the NAND framework does not provide any guarantee
> on the buf passed to ->exec_op() simply because the MTD layer does not
> provide such a guarantee. Reworking that to match how the i2c framework
> handles it is possible (with a flag set when the buffer is known to be
> DMA-safe), but it requires rewriting all MTD users if we want to keep
> decent perfs (the amount of data transfered to a flash is an order of
> magnitude bigger than what you usually receive/send from/to an I2C
> device). Also, I'm not even sure the DMA_SAFE flag covers all weird
> cases like the "DMA engine embedded in the controller is not able to
> access the whole physical memory range" one.

I forgot that this problem was handled at dma_map time (a bounce
buffer is allocated if needed, and this decision is based on
dev->dma_mask).

> So ideally we should have
> something that checks if a pointer is DMA-safe at the device level and
> then at the arch level.
> 
> A temporary solution would be to add a hook at the nand_controller
> level:
> 
> 	bool (*buf_is_dma_safe)(struct nand_chip *chip, void *buf,
> 				size_t len);
> 
> And then fallback to the default implementation when it's not
> implemented:
> 
> static bool nand_buf_is_dma_safe(struct nand_chip *chip, void *buf,
> 				 size_t len)
> {
> 	if (chip->controller->ops && chip->controller->ops->is_dma_safe)
> 		return chip->controller->ops->is_dma_safe(chip, buf,
> 							  len);
> 
> 	return virt_addr_valid(buf) && !object_is_on_stack(buf);
> }
> 
> > I suppose using the CONFIG_DMA_API_DEBUG option could help
> > more reliably to find such issues.  
> 
> Actually, the problem is not only about detecting offenders but being
> able to detect when a buffer is not DMA-safe at runtime in order to
> allocate/use a bounce buffer.