LPSPI3 eDMA max transfer size

[barandas]

Hello,

Linux version 6.6.101
Machine model: NXP i.MX93 11X11 EVK board

I'm trying to develop st7789v LCD display controller using SPI connection through iMX93 evk expansion connector.

I'm trying to use DRM framework. It works fine without DMA, but some performance limitation.

With DMA activated, i'm facing some kind of limitation. The DRM framework is trying to update the whole screen with framebuffer content (size is 115200 bytes), through DMA and is failing as FSL_LPSPI_MAX_EDMA_BYTES is 32K. I was expecting that this kind of big transfer will be split in chunk of max DMA transfer size, but it seems not.

I don't now how to manage this issue. I tried to align spi_max_transfer_size on FSL_LPSPI_MAX_EDMA_BYTES, but it blocks at spi driver probe.

Can someone help on this issue?

Thanks

[Bio_TICFSL · Accepted solution]

Hello,

The maximum eDMA transfer size for NXP LPSPI (Low-Power SPI) is generally 32 KBytes (32 * 1024 bytes), but this depends eDMA controller's BITER register, which limits transfers to 2^15 bytes (32KB) per transfer. The transfer size also needs to adhere to the  bytesperFrame requirements of the LPSPI driver, ensuring it's an integer multiple of bytesPerFrame, especially when bytesPerFrame is greater than 4

 BITTER Limit: The eDMA controller has a limit on the BITER (bytes in transfer) register, which restricts the maximum bytes per transfer. For i.MX devices, this limit is typically 2^15 bytes, or 32 KBytes.

 LSPI Driver Requirements: The LPSPI eDMA driver in the MCUXpresso SDK imposes its own constraints.

• If bytesPerFrame is 4 or less, the transfer size can be any integer multiple of bytesPerFrame.
• If bytesPerFrame is greater than 4, the transfer size must be exactly bytesPerFrame or an integer multiple of bytesPerFrame. 

2.  

Practical Considerations

• Check Your Datasheet: While 32KB is a common limit, always refer to the specific datasheet for your i.MX to confirm the maximum BITER value and eDMA capabilities. 

• Test with Your Driver: Implement and test your eDMA transfer logic with the appropriate LPSPI configuration in your MCUXpresso SDK project to ensure it meets both the hardware and driver requirements

Regards

[barandas]

Thanks