dma_alloc_coherent irqs off too long

don1097 — Fri, 06 Dec 2013 20:58:28 GMT

We are using the IMX6Dual and during startup we are missing characters on one of the Uarts.

We tried using DMA for the Uart and adjusting the receive threshold but no luck.

We noticed that it happens during the insmod of the galcore.ko driver.

Finally ran ftrace for irqsoff and discovered the irqs are off for 1165us and as much as 1700us.

The Uart is running at 230kbaud which translates to 43us/char with a 32 byte fifo. Overrun occurs at 1419us.

Why is this taking so long?

# tracer: irqsoff

insmod-291 0d..1 0us!: __raw_spin_lock_irqsave

insmod-291 0d..1 1163us+: _raw_spin_unlock_irqrestore

insmod-291 0d..1 1165us : trace_hardirqs_on <-_raw_spin_unlock_irqrestore

insmod-291 0d..1 1166us : <stack trace>

=> __dma_alloc

=> dma_alloc_coherent

=> gckOS_AllocateNonPagedMemory

=> gckOS_AllocateContiguous

=> _Construct

=> gckMMU_Construct

=> gckKERNEL_Construct

=> gckGALDEVICE_Construct

Re: dma_alloc_coherent irqs off too long

Yuri — Wed, 18 Dec 2013 02:59:44 GMT

Looks like Linux implementation of DMA API to handle coherency between CPUs and external devices
(when accessing the same physical memory) is not intended for real time applications.

i.MX ProcessorsのトピックRe: dma_alloc_coherent irqs off too long

dma_alloc_coherent irqs off too long

Re: dma_alloc_coherent irqs off too long