Re: [PATCH] fpga: zynqmp: Make word align the configuration data

[Xu Yilun]

On 2023-03-28 at 09:33:17 +0000, Manne, Nava kishore wrote:
> Hi Yilun,
> 
> 	Please find my response inline.
> 
> > -----Original Message-----
> > From: Xu Yilun <yilun.xu@xxxxxxxxx>
> > Sent: Saturday, March 18, 2023 2:55 PM
> > To: Manne, Nava kishore <nava.kishore.manne@xxxxxxx>
> > Cc: mdf@xxxxxxxxxx; hao.wu@xxxxxxxxx; trix@xxxxxxxxxx;
> > michal.simek@xxxxxxxxxx; linux-fpga@xxxxxxxxxxxxxxx; linux-arm-
> > kernel@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> > Subject: Re: [PATCH] fpga: zynqmp: Make word align the configuration data
> > 
> > On 2023-03-14 at 15:12:22 +0530, Nava kishore Manne wrote:
> > > To avoid unwanted copies at firmware(PMUFW) this patch provides a fix
> > 
> > The copy happens in firmware? Please help briefly describe the firmware
> > operations in commit message.
> > 
> 
> Yes, If the firmware receives unaligned Bitstream file from Linux to make them align
> it will do one more copy at firmware and this copy takes much time as firmware code
> runs on microblaze(32-bit processor and runs at lower frequency). 
> So, we suggested the users to handle the alignment issues at top layers(Before submitting request to the firmware).
> 
> Will update the description in v2.
> 
> > > to align programmable logic(PL) configuration data if the data is not
> > > word-aligned. To align the configuration data this patch adds a few
> > > padding bytes and these additional padding bytes will not create any
> > > functional impact on the PL configuration.
> > >
> > > Signed-off-by: Nava kishore Manne <nava.kishore.manne@xxxxxxx>
> > > ---
> > >  drivers/fpga/zynqmp-fpga.c | 15 +++++++++++++--
> > >  1 file changed, 13 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/fpga/zynqmp-fpga.c b/drivers/fpga/zynqmp-fpga.c
> > > index c60f20949c47..70a12dc6e15c 100644
> > > --- a/drivers/fpga/zynqmp-fpga.c
> > > +++ b/drivers/fpga/zynqmp-fpga.c
> > > @@ -15,6 +15,9 @@
> > >  /* Constant Definitions */
> > >  #define IXR_FPGA_DONE_MASK	BIT(3)
> > >
> > > +#define DUMMY_PAD_BYTE		0xFF
> > > +#define FPGA_WORD_SIZE		4
> > > +
> > >  /**
> > >   * struct zynqmp_fpga_priv - Private data structure
> > >   * @dev:	Device data structure
> > > @@ -41,18 +44,26 @@ static int zynqmp_fpga_ops_write(struct
> > fpga_manager *mgr,
> > >  				 const char *buf, size_t size)
> > >  {
> > >  	struct zynqmp_fpga_priv *priv;
> > > +	int word_align, ret, index;
> > >  	dma_addr_t dma_addr;
> > >  	u32 eemi_flags = 0;
> > >  	char *kbuf;
> > > -	int ret;
> > >
> > >  	priv = mgr->priv;
> > > +	word_align = size % FPGA_WORD_SIZE;
> > > +	if (word_align)
> > > +		word_align = FPGA_WORD_SIZE - word_align;
> > > +
> > > +	size = size + word_align;
> > 
> > Does the Macro ALIGN() help?
> > 
> 
> Will fix in v2.
> 
> > >
> > >  	kbuf = dma_alloc_coherent(priv->dev, size, &dma_addr,
> > GFP_KERNEL);
> > >  	if (!kbuf)
> > >  		return -ENOMEM;
> > >
> > > -	memcpy(kbuf, buf, size);
> > 
> > This is historical, but why do the realloc & copy? Any better way?
> > 
> 
> Firmware internally uses the AXI DMA engine to transfer PL data from memory to the device
> and it supports only continues DMA-able memory access(It will not support scatter-gather memory access).
> So, this extra copy is needed to copy the data from kernel memory(allocated by the firmware subsystem using page allocators)

I see, but seems the issues are generic. Same issues for versal-fpga

1. fpga_mgr_buf_load_mapped(), fpgamanager_ops::write are intended for
   contiguous (and maybe dma-safe) buffer but fpga_mgr_firmware_load()
   breaks this. This may also affect spi backend drivers.

2. Some drivers needs dma addr rather than cpu addr, this could be
   generally supported. But allocating a bounce buffer by
   dma_alloc_coherent() is not a good idea.

3. fpga_manager_ops::write() may also be called for each fragment of a
   sg buffer, could the firmware OK to handle this case?

I think framework refactoring is needed for these issues. I may not have
time to do the refactoring but will support these topics in first
priority.

Thanks,
Yilun

> to continues DMA-able memory.
>  
> > > +	for (index = 0; index < word_align; index++)
> > > +		kbuf[index] = DUMMY_PAD_BYTE;
> > > +
> > > +	memcpy(&kbuf[index], buf, size - index);
> > 
> > Generally I object to massive copy in fpga_manager_ops::write if not
> > necessary. If there is an alignment requirement from HW, it should be
> > noticed to the caller in some way, before the buffer is created.
> > 
> 
> Agree, we should find a way to support this kind of use cases. 
> 
> Regards,
> Navakishore.
>